Okay, so a Maga wants to "turn off" one of her own spells early. Say, VEIL OF INVISIBILITY.
First, you can't just "switch off" spells before the duration expires, right? At least, not without a specific virtue. If you can then the rest of this question is kind of moot.
That said, what is the guideline for this? Say you're trying to do this spontaneously. The PeVi guideline is kind of hard to read:
"Dispel effects of a specific type with a level less than or equal to the level + 4 magnitudes of the Vim spell + a stress die (no botch). A specific type could be Hermetic Terram magic, or Shamanic spirit control magic."
...uh, what? Since there's no example spell that I could find with this guideline, I'm kind of lost.
If you wanted to spontaneously dispel a Veil of Invisibility, what level do you need?
Nope, you can't turn off spells without the Harnessed Magic virtue.
General guidelines are indeed a pain. Basically any increases in magnitude on the Range/Duration/Target SUBTRACT magnitudes from the effect.
It becomes easier if you start attaching R/D/T to the spell. The spell is on you, so Personal should suffice (+0M), and Duration is Momentary (+0M) and Target is Individual (+0M)
So your effect will be to dispel a Level of Spell-0M-0M-0M+4M+stress die. If you spont a level 5, you can dispel a 25+Stress die (5 + 4M = 25). If you spont a level 3, you can dispel a 15 (3 + 4M = 15, as 1-5 each counts as one magnitude, then each magnitude is +5). Veil of Invisibility is a 20, a level 3 Pers/Mom/Ind PeVi will dispel it roughly 1/2 the time, a level 4 100% of the time.
If you did this at Voice range, it would be Level of Spell-2M-0M-0M+4M+stress die. So a level 5 would dispel 15+stress die, a level 3 would dispel a level 5.
Or Tamed Magic (essentially gives Harnessed Magic), or Mutantum Magic with a properly designed Mutantum spell, or the Spell Mastery option Harnessed Casting, or D: Concentration (even when maintained by something else based on the comments from Familiar enchantments), or MuCo(Xx) shapeshifting spells with an item built into the casting for this purpose.
Edit: Oops. I mistook general guideline for general spell. Yes, follow John's calculations.
I would simply imitate Unravelling the Fabric of Imaginem. That really is the example spell of that guideline (well, Unravelling the Fabric of [Form] that is).
It dispells an effect with a level no greater than (level)+10+stress dice.
So you'd need a level 10 Unravelling to remove a level 20 Veil of Invisibility.
Or level 5 if you think Range: Touch is enough.
Potentially even less if you care to gamble on that stress die.
Not true, any spell can be temporarily shut down before it expires and then allowed to continue, but you need another spell effect to do it (and some of these effects are truly trivial).
ReVi allows you to "suppress a spell". It's easier if it's one of yours, but even someone else's can be suppressed using such a spell.
It does read a bit like a math word problem...
a level less than or equal to: [the level + 4 magnitudes of the Vim spell + a stress die]
It is all based on the Vim spell itself, so start a bit backwards, start with "the level of the Vim spell" - just pick something at random to get a feel for it, let's say PeVi 10 to start with. 10 + 4 magnitudes = 30, + stress die (no botch)... so a Level 10 PeVi could reliably dispel any Level 30 effect (of that particular type of magic, see Guidelines p 160), and would dispel a Level 35 about half the time.
If you wanted a spell that "dispels" VoI, you could go a couple ways. Cheap would be a PeVi to dispel "Hermetic Imagonem magic", but more useful would be a general PeVi.
VoI is PeIm 20.
o PeVi (Hermetic Imagonem): PeVi 4 (yes, "four"*) + 4 magnitudes + stress die = (Level 20 + stress die) - done. Easily sponted by (most) any mage.
(* thx Cal!)
o PeVi (any magic): Half the (Level +4 magnitudes + stress die) - so we need an end result of 40/2 = 20, so PeVi 20 (b/c 20 + 4 mags + stress die = 40+/2 = 20+).
Otoh, if you just wanted to "switch it off" so you didn't have to recast it (handy if the effect is from a diff mage, or an item w/ limited times/day), you use the ReVi effect, "Suppress an effect"... (no stress dice are involved in these Guidelines)...
o ReVi (that you have cast): LESS THAN Level + 2 magnitudes; so base ReVi 11 (since PeIm 10 < 11+10). To suppress an effect for Concentration duration, ReVi 16.*
o ReVi (cast by another): LESS THAN [Level + 5 mags] /2; so since 20 < 41/2, we need a base ReVi 16* (16 + 5 mags = 16+25 = 41, 41/2 > 20). To suppress an effect for Concentration duration, ReVi 21.*
(* Note - Although there may(?) not be canon examples of "odd numbered" effects above Level 5, there is nothing to say they cannot exist. In most TeFo combos they are unnecessary, but with many Vi effects, and Wards, and "contested" effects such as Intellego where two spells are compared, smaller differences become significant. ysmv.)
I looked, and nowhere does the OP use the word "end" or "dispel", except in quoting the rules, and that could be taken a couple ways.
What he actually does say in the opening statement was "switch it off" and "turn off" - and by "switch" I took that to mean it could also be "turned back on". (Which is how a "switch" works, and why one might choose those words over something else like "end" or "dispel".)
But meanwhile, since I included both - what exactly is your quibble?
(rhetorical question, btw)
Urgh - serves me right for posting before the coffee kicks in. (Those did seem a little low at the time...)
My reading of this is quite different than most posters here, I think.
If you are mathy, this may help-
Dispel a magical effect <= Level # + 4 magnitudes + stress die rolled number.
Therefore, if attempting to dispel a spell level 20 you must equal or exceed (20 + 4 magnitudes + random number). You need at least a 40 to overcome a 20, plus a random variable.
Compare Wind of Mundane Silence, p. 161, where you overwhelm a magical effect, by generating a total of twice or more the target effect with the level of your spell plus a stress die, (Level of spell plus rolled number).
Also, look at the 4th guideline of the PeVi chart, "Dispel any magical effect with a casting total less than half the (level + 4 magnitudes) of the Vim spell + a stress die (no botch)." To defeat a Level 20 spell, you must beat (20/2 = 10) + (4 mags) = 30 on the Vim spell + a stress die.
In all of the Perdo Vim example spells, you must exceed the target's level or might.
@TimOB, I think you're doing the calcs in the wrong order, because the language is written in terms of the Perdo Vim spell being cast, not the effect to be cancelled. It seems to be written so that the person casting the Dispel can work out their casting total + roll against the potentially unknown level of the spell they are trying to affect. And remember too that the spell is further modified by the casting conditions beyond the baseline rules, by modifying the magnitudes listed in the formula.
It also has the tricksy "half the level of" part because of the other General Perdo Vim guideline for specific spells, which works on the full level of the Perdo Vim spell instead of half the level of the Perdo Vim spell. It makes sense that it is easier to dispel a narrower range of magic with specific spell.
But if you're right about the order of the math then PeVi just got a tad harder to do.
Or to say another way, the PeVi baseline spell can dispel any effect which is equal to the PeVi level divided by two plus a stress dice. A PeVi20, is cast as (Pe Vi effect level 20/2) + d. Then compared to the level of the spell to be cancelled. That interpretation ignores the actual use in casting use though.
I read this as:
Level of the PeVi + 4 mags. So Level + 20 at this stage.
subtract any modifiers for Range, Duration, Target to the magnitudes (such as Voice +2). So (Level + 20) - 10 at this stage.
then half that result. So (Level + 10) /2 at this stage.
then add a Stress dice. So ((Level + 10) /2) + roll at this stage.
then compare to the spell to be Dispelled.
So to dispel a level 20 Illusion with a general "dispel magic" style effect, you might use a level 20 casting, at range Voice:
so its (20 +10)/2 + roll
which is Effect level 15 + roll vs Illusion's level of 20.
If another spell was designed just to dispel other Hermetic Wards at level 20, then it can use the specific spell guideline:
so its (20 +10) + roll
which is Effect level 30 + roll vs whatever the Ward's levels are.
Guideline level ≥ Effect level to be dispelled - 4 magnitudes - stress die
So the higher the guideline the more it can handle. It's not about underwhelming the target effect. Higher guidelines are better at dispelling. You just put the 4 magnitudes in the wrong direction. Also, you had the same error in moving the stress die to the other side. You solved it as:
Guideline level ≥ Effect level to be dispelled + 4 magnitudes + stress die
As you can see, it's not a shift of ≥ to ≤. It's a shift of +4 magnitudes to -4 magnitudes. That commonly makes the dispel effect for a specific type of magic 40 levels lower than you were calculating. Well, when including the stress die about 50 levels lower. This allows you to build a dispel effect with some range and maybe T: Group or similar and be able to dispel effects comparable to its level instead of requiring the dispelling of even minor effects to require ritual magic.