Hi,
I tried to figure out how to design a formulaic spell intended to counterspell. This spell would obviously be Pe Vi, and would need a mastered ability of at least 1 to take the mastery capacity: fast casting. Thus it would be able to react to other’s spellcasting and counterspell efficiently. Is such a spell within guidelines? How would you design it? Thanks 
Look at winds of mundane silence. it might be what you are searching for.
Cheers,
Xavi
Winds of mundane silence might be a spell of a level too high. Remember that you need to double the target spells level (IIRC, serf's parma)
Spells like unravelling the fabric of (form) might be better. However you need 10 versions of the spell to cover all forms.
This is the spell you want.
PeViG Thwarting the Thaumaturgical Threat:
R: Voice, D: Mom, T: Ind
Similar to Winds of Mundane Silence, but scaled down to affect a single spell. This will counter or cancel a spell if your roll of a SD + (Level + 10) equals or exceeds twice the level of the opposing spell. Thus for a Level 15 version, the roll is a SD +25 versus an ease factor of twice the level of the opposing spell. Countering another spell as it is being cast requires a successful Fast Cast roll, and thus requires Mastery of the spell and choosing the Fast Cast ability. Canceling an existing spell uses normal Initiative, and no Mastery is required.
(Base 5, +2 Voice)
It also runs afoul the law of arcane connections - If you can't perceive the spell you can't target it. The caster would want to have some sort of a long duration intellego vim spell up and running if they want to cast Thwarting the Thaumaturgical Threat with any reliability (Wind of mundane silence which targets a room does not suffer from this problem but there is some question regarding whether or not you can use it outside.)
Killjoy!!!
Someone is casting a spell at me, I am aware of it as soon as I can hear the words and see the gestures.
If you can hear the words and if they use gestures you mean.
Or if I can somehow percieve the manifestation of the wizard's sigil. But otherwise yes, if I cannot percieve a spell or have know knowledge that one is being cast, then I cannot target it as it is cast. That would require some form of precognition.
There is no counterspelling in 5th edition, it doesn't exist (in the core book at least)
The word "counterspell" doesn't appear in the index, and it is not mentionned in the rules for fast-cast defence.
The only thing that remain, is fast-cast defence.
For exemple, if your opponent throw a Crystal Dart at you, you can invoke a Wall of Protecting Stone to stop it, of destroy it with a PeTe spell.
I would say that a MuTe could be used to return the Crystal Dart to the earth where it belongs, giving an effect equal to a counterspell in 4th ed. (so in this way, counterspell is still possible, but is the same as fast-cast defence)
I wouldn't allow for a multi-use, formulaic, Perdo Vim spell to be able to stop any and all spells, as it would unbalance game play IMO.
Also, the rules about fast-cast defence (p.83) would suggests most of the time such defences will be done with spontaneous spells.
Your PeVi spell could be used to dispell any spell that have a duration longer than momentary, still very usefull.
Okay, you do know that your position is in absolute contradiction to the RAW. I can Fast Cast a defense, which can be a spontaneous or a mastered spell. It targets the opposing spell, just like any other Fast Cast defense. AND, specifically as per the rules of Dimicatio in Societates, Vim spells do indeed count, but they must use all the normal rules for vim spells. The very essence of the Form of Vim is targeting other spells with Fast Cast Vim spells.
And please note that this is a far less effective meathod than using a proper and precise counter spell (opposing Ignem with Aqual for example). In the later case, the defense only needs to e half the level of the opposing spell. With this Vim spell, I need to toll twice the level of the opposing spell with a SD + Level + 10. That can get quite difficult from my experiences with my level 15 version of the spell. It works on level 15 or lower, I have little hope of countering a level 20 or higher spell. And it is not a "counterspell", it is a spell used to counter another spell. Semantics, I know.
And you have to Fast Cast this spell in order to counter another as it is being cast, otherwise it is just a dispel spell. That means it must be mastered.
Your game may vary, but this spell is squeaky clean RAW and not at all unbalancing. A Fast Cast Winds of Mundane Silence already can do this. Nothing new, nothing imbalancing. I know you wouldn't allow it, but as I recall, you don't allow magi to Fast Cast or do a lot of things that are normal RAW.
Unconceivable as it may seem, I find myself agreeing with Marko here... 
Ah! it's been so long that I've not Cross blades with you =) [thanks to Fruny for translation of "croiser le fer"
]
No, we don't allow mages to cast spells at all, since we found this is the most unbalancing rules of all. 
Seriously, I think my way of reading the RAW is correct. See for yourself.
Check fast-cast defense against magic in p.83, it's in the spontaneous casting section. I always seen this as meaning those kind of defenses needed to be tailored to the specific circumstance to be effective. (in the same way as MuVi effects coming from a magical object can't target a spell outside that object).
About game balance:
Well, your spell at level 20 would dispell spells of level 15 with a casting score of only 10,
Where the rules demands a spontaneous spell that need a casting score of 30 to acheive the same goal. Notice that a casting score of 15 would only redirect the spell, or protect only one person, and does not counter is entirely. This is still 5 more needed than your formulaic-all-purpose-PeVi-spell.
Yours at level 40 dispell level 25, need casting score of 30, rules demands casting score of 50 for 100% dispell and 25 for partial dispell. It's similar, but in some circumstances yours will be a lot better (no fatigue,etc). Why a mage would not have that one particular spell, I cannnot say.
With your spell, you don't need to know what is the technique/form,
where the rules demands awarness roll to discern that to make defence possible.
With your idea, wouldn't any mage be able to learn a PeVi spell specific to ignem to counter any and all ignem spells ? (similar to Unravelling the Fabric of (form)) And that spell would need only to score "One time" the level of the spell to be 100% countered. With a level 40 PeVi I could counter all Ignem formulaic spells ? O.o
Our troupe don't like spell ideas that are so good that it become difficult to imagine every mages without it. Maybe that's why saw that (or another reason)
("crossed steel" 
)
Some of what you say is in the RAW, but you are reading more into it as well.
As I understand the rules, any spell needs a target - that is, if you cast a PeVi spell, spontaneous or otherwise, there had better be a Vim-based target for it to Perdo, or it's wasted. Until a spell is cast, there is no Vim target - and after it is cast is too late, and there are no RAW for "perfectly timing" a formulaic spell in such a manner.
To use a fast-cast-mastered formulaic spell as a defense, the effect must be "appropriate" to defeat the other spell - a quick Wall o' Stone will stop many effects, but not all; a Wizard's Leap can get you to another place before the spell targets your location (unless it's targeting via Arcane Connection), and so on. Formulaic spells like this are guesswork, but rely on the effect arriving first.
But when you use a fast-cast Spontaneous spell, the effect and time are undefined - it's just "exactly the right effect", based on the Perception roll. And it's assumed that, as the correct effect, that it's timed perfectly - that's all taken into account. A formulaic spell is rarely "exactly the right effect", much less with the perfect timing.
"Counter spells" work against existing spells - there is no single momentary* spell, in canon, that blocks a spell as it's being cast. (* As mentioned above, some PeVi with duration could do the job.)
I may be missing something - all the relevant info is not in one tight section - but at the moment I'm confident that's how it works by the RAW.
(That is, "I'm sure, but that's not a guarantee that I might not still be wrong." 
)
I almost said that, but thought it would be too rude. Different styles, i understand. But you should understand that your interpretation is a stylistic preference, not the RAW
And for Fast Casting Mastered spells, it says to refer to that very same section. Your inferrence is not what it says, it is a stylistic preference. You are adding what isn't there.
Casting score is irrelevant. If the spell succeeds, it has the stated effect. Just like Winds of Mundane Silence.
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Where the rules demands a spontaneous spell that need a casting score of 30 to acheive the same goal.
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Incorrect. You only need a spell half the level for that sort of defense. So to go against a level 20 spell, you need a casting score of 20 for a resulting spell level of 10, and no secondary roll is needed.
You did the math wrong, see above.
Level 40 to counter or cancel a sinlgle effect three magnitudes lower, and you call that unbalanced??? And your math is still wrong. The spont spell only needs half the numbers you mention.
There is just too many good spells, you can't have them all. And you never thought of it, I did, so I have it and you don't . That's just the way this game works.
Again I will refer you to Societates and the House Flambeau section Perfectly legit. It is very much harder doing it the rough way like this.
Yes, precisely, exactly. Mind you, that takes 10 different spells. And, in fact, "Unravelling the Faric of (Form)" is exactly the spell to use for this, so long as it is mastered and fast cast. My new spell is nothing more than a scaled down version of Winds of Mundane Silence. The +10 bonus comes from the fact that it is Target-Individual as opposed to Room. Unravelling the Faric of (Form) is already scaled perfectly, and has the proper range. That is the eact spell to use, and that is indeed a legitimate use for it.
That is all well and good, but you need to keep that in mind when discussing the RAW. Your stylistic preferences are ot what the rules say. That's all good. I have my own stylistic preferences, and I know to keep them separate when discussing raw.
And almost all spells are so good that every magus needs every one of them. It is just not possible to do that though.
(crosspost, Mark - see above) 
Yeah there is, just the same as Muto Vim on an uncooperative magus. (see MuVi guidelines). And Fast Casting is specifically the rule it specifies in order to get the timing right. Mind you, this is a two-part preparation and a three roll effort. I need to invent the Formulaic spell, and then I need to spend additiona; effor to Master it, choosing the Fast Cast Ability. Now, when I go to use it, I need three successful rolls in succession; a Fast Cast roll (for if I am short by even a point, I am out of luck), a Casting roll (with a -10 penalty for Fast Casting), and then the actual effect roll (SD + Level + 10 versus twice the level of the opposing spell). All three are stress rolls.
How is Perdo Vim not appropriate?
Not sure about that second one, since there is no Targeting roll, as long as the magus can still see you and his voice reaches, the spell still gets you. Otherwse I would have relied on my already mastered Wizard's Leap for the Dimicatio.
I dispute that, because all spells are targeted by Vim, regardless of their Arts.
I will spell this out again. It is a bbrutal and corse effect, much more difficult than using the "appropriate" counter. And the rules for Dimicatio in Societates specifically adress the use of Vim counter spells, stating that they must abide by the normal rules rather than having the advantage of the "appropriate" spell.
Yes there is. Winds of Mundane Silence or Unraveling the Fabric of (Form) can do the exact same thing if they are Mastered and Fast Cast.
(clash of steel!)
You would be wrong. You failed your roll . Look up Dimicatio in Societates.
Methinks I have drawn blood!
Dude, you are just simply incorrect. Usually it is you that burns me for casual incorrectness, which is why I put up the Perpetual Smurf's Parma , but I have you on the ropes this time, my worthy advesary
I agree with Marko here. A fast cast perdo vim spell that is applicable to the target spell to me is a perfectly viable counterspell.
Hmmm - that is true, and I had missed that... BUT... (:lol:)
Muto Vim Guidelines specifically state that MuVi effects work differently than any other combo, including Perdo Vim. They have unusual Restrictions - for one, they cannot affect spontaneous spells, which PeVi can. More, they cannot affect spells after they have been cast, so it's unclear if those rules can apply to PeVi - which only affect spell effects.
(I'd think that a fast-cast Vim spell could work against spells of the Form of Vim, as with any other, since that is the largely the only way, by either of our understandings, one could defend against such.)
I think the most compelling argument in the RAW is under the Spontaneous Spells section - the fact that nowhere does it say "Oh, btw - PeVi stops anything!" Nowhere in the book does it even suggest it as far as defensive spell vs. spell magic. (And you'd think they would, it'd kinda be important.)
As pointed out above, the term "counterspell" is never used in AM in this context or any other - it's being imported from another game. The Fast-Cast rules talk about casting as a "defense" against other spells, or "in response to them" - never as a pre-emptive strike that prevents them from being cast in the first place. (Tho' see below...)
With one exception (see next), you can only Perdo the effect, so you must wait for it to be cast - and once it is cast on you, it is too late, as at that point you've been affected. And in the fastcast rules, it never discusses targeting the casting, only the effect.
Don't have Societates (so I'll have to let someone else speak to that, and keep my fingers crossed), but I'm glad you brought this up. I'll now offer evidence on your side - just not the same case your making...
By the PeVi Guidelines (part of "the normal rules" of PeVi), the only base effect that could work (I did find one for you!) is the following:General: [list]...
Reduce the casting total for all magic cast by the target by half the (level +2 magnitudes) of the (PeVi) spell... The spell must penetrate the target's Magic Resistance in order to have any effect.[/list:u]So, if you can successfully fast-cast that type of spell, and beat (or equal?) the other mage's initiative, and Penetrate, and lower their Casting Total to where the spell doesn't go off... problem solved.
(If this is what you were referring to, my bad, and I'll eat much crow for borfing - my Matho technique has not been strong lately. Apparently I took the cyclic Flaw, and I'm in a down swing.)
But since any other PeVi effect doesn't work until the opposing effect already exists, I just don't see any of those working - short of a general Rego Ward against Magic, or fast casting a spontaneous ReVi type deflection with at least a Momentary duration. By the RAW, you can't cast a (non-ward) spell without a Target, and with that one exception, PeVi targets effects, not the casting of spells before an effect exists.
Looks like i didn't use the proper word when i said "countermagic".
I actually meants fast cast perdo vim defense to dispel the targeted spell before it reach its own target.
Since it's possible to fast cast Creo Aquam or Perdo Ignem to extinguish (or at least mitigate damages) the Ball of Abysmal Flame, or Re Ig to change the target or even Cr Te to build a solide cover, what would prevent a Pe Vi to destroy the magic of the fire ball?
If i remember correctly, the Ball of Abysmal Flame appears somewhere near its caster and run to its target. Flames in this ball are magically created (and maybe they are also magically directed, though there is no Rego involved). So, Pe Vi should be able to dispel the magic creating the flame?
Pe Vi seems to me just a much more universal (save it as to target a specific form, as pointed out) way to have correct fast cast defense. And since it's much more universal, why not make it a formulaic spell and get the fast casting ability through mastered spell?
You don't remember correctly. Not sure where you think you read this, but at best it's a "cosmetic" effect. (In 5th ed at least - 3rd ed might have worked that way.)
The main problem is that the examples you list are elements, and solid (to one degree or another) - Vim is abstract. You can create a momentary Wall of Stone and stop anything flying your way - but you can't create a Wall of Perdo Vim that does something parallel.
For most of the forms, it is admittedly vague - there's "stuff" coming, and so you counter with a reasonable "other stuff". But countering with "I destroy all magic" is simply not the same, not in feel or game mechanics. If it was that easy, the rules would have mentioned it at least in passing - and yet there's nothing even close.
But like I say, if you want to, go ahead and houserule it - but I still ~believe~ (short of some new info from Societes) that it goes against the RAW.