Tetrica said:
Not really, what's confusing (after learning the values for melee/spell-multipliers) is the part where some values add and some multiply.
As a whole if a talent or whatever increases the damage of an ability, simply add all these percetage increases together and do the sums. If however there is a flat increase (such as AP or Spell Power) then you do these calculations on the base effect of the spell first as all % increases affect total damage (including AP/SP). Thus for Exorcism if we consider all talents etc possible to increase it's damage:
20% from glyph
15% from the talent
6% from Crusade
3% from improved Ret Aura
9% from Vengeance
This is a total increase of 20 + 15 + 6 + 3 + 9 = 53% extra damage.
Exorcism takes 15% of your AP and 15% of your Spell Damage and adds to the base amount (250 - 283 base damage). Assuming 600 AP and 0 Spell Power:
600 * 0.15 (AP co-efficient of Exorcism) = 90. Add this value to the base damage of the spell equals to (340 - 373 damage). THEN we apply all talent effects to this value (53% extra damage remember):
(340 * 1.53) to (373 * 1.53) = (520 to 570 damage).
Finally, if we assume the spell crits (and because it's a spell this will be 1.5x the damage) then we simply multiply our damage range by 1.5:
(520 * 1.5) to (570 * 1.5) = (780 to 855 damage).
Simple as that
-X
P.S. If you wanted to add in zerker damage or a warrior in zerker stance etc just add up all the totals (assuming a zerking warrior, in zerker stance with death wish on, and you also have zerking) = 10% + 5% + 5% + 30% = 50% extra damage.
(780 * 1.5) to (855 * 1.5) = (1170 to 1283 damage *nerf!*)
The same way you can also factor in damage reduction effects (say a Warlock with glyph'd Soul Link spec) would be 25% reduction:
(780 / 1.25) to (855 / 1.25) = (624 to 684 damage).
P.P.S. This damage reduction you have to calculate with melee attacks as everyone has armour which reduces damage by x% and gives you more accurate results in theorycrafting. Whereas very few people run around with spell resist sets (or practically none). Soul Link is the main one you have to factor in for spells