Originally Posted by
cynara
I'm not sure what exactly you're trying to ask. For a priest, magic damage can be present directly on an item, or it can come from wisdom, where each point of wisdom gives you 0.6 points of magic damage. So from that point of view, the formula is simply [effective magic damage] = 0.6 * [wisdom] + [direct magic damage]. This effective magic damage is what I used in the comparison linked to in my previous post.
But if you're asking how much does a piece of gear contribute to the actual damage that you'd do, on average, in some sort of fight, that would be a lot more complicated to model, it would depend on the fight and your spell rotation (how does your magic damage increase the amount of damage actually done by a spell? this turns out to be a linear function, but its slope is different for each spell), etc. You'd need to take into account other stats on the item(s) that also affect your damage, e.g. things like hit rating (which decreases the chance that the enemy will resist your spells), critical rating (increases the chance of critical hits), critical damage rating (increases the bonus damage that you do when you make a critical hit).
And these stats interact with each other, so a formula couldn't really evaluate an individual item without knowing something about your character's existing stats. For example, an item might have some critical damage rating. This increases the damage of your critical hits, so the amount of damage that this really gives you depends on how often you crit (i.e. on your critical chance, which depends on the rest of your gear, possibly your feat build, group buffs, etc.).
Likewise, if an item has hit rating, this might be a useful contribution to your DPS if you're fighting an enemy that has a high amount of immunity (e.g. a high-tier raid boss), but if your hit rating from the rest of your gear/feats/etc. was already high enough for the enemies you were dealing with, then increasing it still further won't make much of a difference.
As a general idea, and ignoring the issue of hit rating and having the enemy resist your spells, you could afaik try to go with something along the lines of [damage done] = (C1 + C2 * [effective magic damage]) * (1 + [critical chance] * (0.5 + [critical bonus damage])), where the constants C1 and C2 depend on the spell you're looking at.