When it says
does the level relate to the base level of the spell?
I.e. as PeVi30 it can dispel spells of the first magnitude. (Base effect, +2 Voice, +2 Room) would add up as 2*5+10+10
If it does relate to the total level and dispels magic up to level 15, how would one calculate different target or range?
For Wind of Mundane Silence it refers to the total level of the spell. It is based on the guideline "Dispel any magical effect with a level less than or equal to half the (level + 4 magnitudes) of the Vim spell + a stress die (no botch)." from page 160.
Wind of mundane silence is range voice and target room, it dispels targets at half the spell level + a die roll, because the spell level is 4 magnitudes higher than the base level.
I think the calculation should be: base PeVi effect Twice the (Level +4 mags), and then the Room Voice subtract 4 mags. This means it does twice its level.
It does get a little ugly with these doubling/halving and adding magnitudes. I'll go through all the details in two ways to show how it works and to show how to make it simpler to use in the end.
The easiest way to figure these out is to determine the base needed to do what you want, and then add Range, Target, and Duration as needed. For instance, let's say I want a R: Sight, T: Individual, D: Momentary version of this spell that dispels at least level 15 spells and maybe up to level 20 (or higher if we're really luck spells). We look at this guideline:
So 15 = 0.5 x (base + 4 magnitudes), or 15 = 0.5 x (base + 20). That gives 30 = base + 20, or base = 10. So we start with a base of 10, and then we add 3 magnitudes for R: Sight to get to 25.
The way you would write this spell to make it easier is to go the other way. The base + 20 (the 4 magnitudes) + die roll needs to double the level of the spell being targeted. Since the base is the spell level - 15 (3 magnitudes for Sight), we can change it to the spell's level - 15 + 20 + die roll, or spell's level + 5 + die roll, needs to double the level of the spell being targeted. And what I actually do when writing the particular spell a character knows is actually plug in the spell's level so I only have to worry about the die roll. So inventing a level-25 version of this would dispel a spell if 30 + die roll doubles the level of the spell being targeted.
Actually, Room subtracts only 2 magnitudes, but so does Voice.