Can a Rego Vim spell be used to essentially dispel a spell as it is being cast, simply by suppressing it for a longer duration?
Suppress the Incoming Spell (ReVi gen)
R:Voice, D:Diam, T:Ind
Suppresses the effects of a spell cast by another magus. If the target spell's duration ends before it is released, it dissipates without further effect (''Suppress the Incoming Spell'' does not maintain it). This means that, cast in a timely fashion (higher Initiative, or through fast-casting this spell), ''Suppress the Incoming Spell'' can be used to block any spell with Diameter or Momentary duration. The target spell is only suppressed if its level does not exceed half of the (level + 2 magnitudes) of ''Suppress the Incoming Spell''.
(Base effect, +2 Voice, +1 Diam)
Using the ReVi guideline "Sustain or suppress a spell cast by another with level less than half the (level + 5 magnitudes) of the Vim spell." (ArM5 p.161)
This spell essentially uses "less than or equal" instead of the strict "less than" stated in the guidelines, because the ReVi spells in the MRB all seem to do so.
Would a D:Mom be sufficient to block a D:Mom spell so that no effect actually occurs?
Which is much easier than the ReVi guidelines suggest.
Would it be possible to design a formulaic spell to provide a similar defense? It could then be mastered for fast-casting and replace spontaneous magic for a magus that cannot do spontaneous quickly or well (because of a flaw like Mercurian Magic or Weak Spontaneous Magic).
Unfortunately, I think the spell must be voice, not touch, unless it is a cooperative situation and not a conflict.
What's the +5 magnitudes there for, I'm wondering if it's there to cover +2 Voice and +1 Diam and +2 for what... Group, I dunno! If it's T: Ind, the +5 more than covers magnitudes in excess of R:voice, D:Mom, T:Ind (by 3 magnitudes, to be precise).
So, a ReVi Supress the Incoming Spell, Level 45 R:Voice, D:Diam, T:Ind would be sufficient to block a BoAF, IoL or lesser spell.
Keep in mind, that the way the formula works, that a 40th level spell is effectively unable to be blocked with this guideline, since it pushes the level of the necessary ReVi spell to 55th level, which would be a ritual...
Effect level needed to block a level 35 spell = 45
R:Voice makes it 55
D:Diam makes it 60
Increased range and duration must be added. So the (level + 5 magnitudes) effectively becomes (level + 2 magnitudes) with R:Voice and D:Diam.
So the maximum that a non-ritual version can block is level 30 --- half of (50 + 2 magnitudes).
Which is why I asked about partial deflection.
If I compare what the guidelines indicate and compare this with what the fast-cast defense section says, this would seem to indicate that if you only deflect the spell (protecting an individual target but having to deal with side effects), you would not have to divide by 2. So a level 20 ReVi at R:Voice would be able to deflect a level 35 spell. [[ (level + 3 magnitudes) ]]
IMO, I don't think you need to add the range and duration, as they are effectively included in the +5 magnitudes described in the baseline. Yes, it needs to be included in determining the spell level, but adding them to the spell above and beyond the base is penalizing yourself needlessly.
Look at Maintian the Demanding Spell and compare it to the guideline: "Sustain or suppress a spell you have cast whose level is less than or equal to the level + 2 magnitudes of the Vim spell." The spell itself, includes +1 Diameter and +1 Touch in it. It's not getting added in twice...
A) Once the spell reaches level 50 (or 55 anyway), it becomes rather mood, being a ritual.
B) As for the main gist of the talk... I'm sure I've seen this discussed before, now if only I could find it...