This seems an unusually ... 'open-ended' Flaw. "If you fall short of or exceed that target by more than one level, the spell still works but its effects are beyond your control"
A particular kind of person might then attempt a ridiculously high-level effect, knowing that failure is inevitable but, according to the text above, "the spell still works ...". Any particular thoughts on that? I don't know that I want the character to be some kind of 'open conduit for magical effects' when others who fail spontaneous magic simply fail to make anything happen at all. At the least, I might consider a ruling so that, the more the dice roll falls short of the target, the more likely that the magic will affect the caster badly.
Spontaneous Magic cna be tricky to work out, even under normal circumstances. A lot of people can't grasp what the mechanics are. I've had players who work out the Tech Form combo, rolle a die and sit around looking like a lost puppy until someone works out the magic or decides what happens.
Chaotic Magic can be a real pain.
Now, for Spont magic to work well I need to have the player decide on the Tech & Form, the guideline aimed for, the Range/Duration/Target parameters and whether to spend fatigue. Oh, and also any Penetration you want, however we reraly see this done. If you need to rely on the Pen of Spont magic you migth already be screwed. Our convention is to allow a single parameter to stay open, in case you roll very well. This may be Pen So for CrIg either "the brughtest light possible with Touch/Diam/Ind" or "light as a torch at Touch/Ind for as long as I can".
And if you fail to meet your stated goal the magic may or may not work. In the first example above, if you fail to roll high enough too get any light at all at Touch/Diam/Ind nothing happens IMHO. I mean, if the final level with magnitudes subtracted for parameters takes you below the base level guideline for the effect you want your magic fails to work.
In the second example you might also roll so poor that nothing happens, but using a guideline sufficently low compared to your Arts you may get some limited effect. I.e. your light may appear only momentarily.
But with Chaotic Magic you loose control if your final level differs more than 1 level (above or below) your stated goal. So IMHO you can't leave parameters open, because this means you have no set goal. Your magic still goes off, IMHO even if you roll very low, because this is a Flaw, in fact a Major Flaw!
Note that casting non-fatigueing spontaneous magic does not add a die into the equation, plus the Casting Total is Casting Score/5. Hence there should be little surprise about what level you can cast. Most other modifiers are reasonably clear. Of course, you may be unaware of the Aura, but this is often something you can use magic to at least get an idea of.
Yes, I follow what both of you say with interest. The main thing that I think I must bear in mind is that this is a Major Flaw. So it absolutely should be a real problem. I think that I'm a little daunted at potentially having to continually come up with interesting "how it goes out of control" descriptions (the character with this flaw is also Overconfident) but I should just toughen up and get on with that! I think that I'm tempted to draw a distinction between "fails by more than 1 level" and "succeeds by more than 1 level". Both should be out of control but the "succeeds" one is more likely to achieve something of what was intended, even if the CrIg effect then goes on to burn the local cathedral down, etc.
To digress mildly, an entertaining difficulty with this new campaign is that I have three pretty much new-to-Ars-Magica players (my wife is very familiar with the background and the feel of the game but is avowedly "not a rules person"). Of the three characters, one of them has Diedne Magic (the aforementioned wife) and another has Chaotic Magic. Woo hoo! Multiple Spontaneous Magic formulae are needed! In addition, the players will want to know: "Well, what's my chance of actually achieving this desired spontaneous result?" I have therefore applied technology and programming skills to the problem and created a little application where I select the character, the arts they are using and the aura they are in (all of their Art scores are stored in an accompanying file) and it throws back all of the scores for stress dice rolls up to 40, taking into account requisites, forbidden magics, etc. And tells the player with Chaotic Magic "these are the numbers you want to roll on that dice". I am tremendously pleased with myself.
Is the question really what effects could happen when they keep trying?
Having played a character with the flaw I can't every think of a situation when I was casting a spont spell that I did not want the effect to be controlled( but I was not a combat magus). I do know that you give up the chance for exploding die rolls with this flaw. This takes away the possibility of increasing the magnitude or adjust the range or target(Arm 81-82). Not quite getting the total normally produces a much weaken version of the spell if anything at all. ArM5 book gives an example of creating a light spell that when the coating roll is missed it makes a weaker light or not up to the duration. So getting to within 5 of your target, on either side of the target, allows for control of the spell even if it does not have the full power.
If we first assume that anytime the magic is beyond their control it is never beneficial to the character. That makes things much easier and during play, once a character has had their shield grog killed by them or they burn down the village or make all their party's metal disappear, they may think really hard about the flaw.
A bit of thread necromancy for some clarification on Chaotic magic as I couldn't see my answer in a previous thread.
The key part of the Chaotic magic flaw
"If you fall short of or exceed that target by more than one level, the spell still works, but its effects are beyond your control."
There's 2 points I'd ask for some opinions on.
Casting something ridiculous. One could spont a level 50 spell, fail the roll by 90 but RAW the spell still works.
I think the logical answer is the SG says No for attempting ridiculously ambitious spont spells.
Failing the roll by 2 or more.
Without a -3 flaw, the spont spell would fail. It seems a bit wrong for a flaw causing a good outcome sometimes. Should a roll which fails to cast the spell normally, nearly always have a bad result with chaotic magic, while if the die roll succeeds by too much,the result generally be weird and not necessarily the desired outcome, but not bad?
I had to adjudicate on this last week, when the mage tried to spont a Touch range PeHe to get through an iron bound oak door that had rusted shut. (Botched the roll in an Infernal Aura)
So I said the wood broke violently up in such a way that the rusty metal components fell hard on his character's foot. Rolled for damage and the mage was limping on a bloody broken toe for the rest of the adventure. And told the player that we are going to later roll to see if his mage caught tetanus.
Which means that I hope tetanus is in Art & Academe, or I will have to invent it.