Originally Posted by vorda
100 int = 1500 mana
1500/(6*12)= 20.8 mp5
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That is not entirely correct, of course. To be more precise, 100 int = 1500 mana + 1.25% spell crit chance (at level 70).
For any holy priests who stack Int in part because they love healing spell crits, it may be useful to mathematically discuss why using this aspect of Int is a very inefficient way to improve your healing efficiency. I think one reason people rarely discuss the exact effect of +spell crit on healing efficiency is the difficulty of calculating its mana equivalent. I believe the general equation for the MP5 equivalent of 80 Int (= 1% spell crit at level 70) would be this:
MP5 = (((CHPM * ACH * (1 - OH) * .005) + InspirationFactor) * AMHP) /12
where
CHPM = the caster's average crit-able heals per-target cast per minute (with one PoM or CoH counting multiple crit-able heals). This value can vary dramatically depending on the spell repertoire used.
ACH = average base health points healed per-target per crit-able heal. This value can vary dramatically depending on the caster's +healing, talents and spell selection.
OH = average percentage of overhealing
AMHP = the caster's average mana cost per health point healed. This value will vary dramatically depending on the spell repertoire used.
InspirationFactor = amount of health points not required to be healed due to additional procs of Inspiration resulting from the additional 80 Int. This value can vary dramatically depending on the target's armor value, the DPS being absorbed by the target, and the frequency with which Inspiration procs overlap (whether with your own procs or other casters').
So it's no wonder that it's just easier to ignore spell crit when theorycrafting Int for healers. But out of curiosity, what would the MP5 equivalent of 80 Int look like for a well-geared, caster in an ideal critting situation?
Assume a holy priest with +2000 heal buffed and 5 Points in Mental Agility. Assume he is spam-casting CoH rank 5, with no casting delays and no overhealing, on a 5-man party. For now let's assume he does not have Inspiration. In that scenario I believe the above equation would be:
(((200 * 811 * (1 - 0) * .005) + 0) * 0.1) / 12 = 6.75 MP5 = .084 MP5 per Int
Thus, ignoring Inspiration, in this ideal, rosy-lensed scenario the 80 Int would be generating 6.75 MP5 equivalent from the additional crit healing. Considering the utter unlikelihood of this perfect critting scenario ever occurring, the MP5-equivalent contribution of Int's spell crit component does seem negligible for anyone without Inspiration.
Now let's also assume the healer has 3 points in Inspiration, that every Inspiration proc does not overlap, and that his target party has average armor of 16000, and is taking average melee DPS of 1000 from a level 73 boss. Each proc of Inspiration thus increases the target's armor from 16000 to 20000, for an additional damage reduction of 5.35% (using armor formula from Wowwiki). 80 Int contributes an additional 2 procs of Inspiration per minute (1% of 200 targets being hit per minute). Thus, 80 Int contributes 30 seconds of 5.35% reduction of 1000 DPS, i.e., a total of 1605 hit points. Now the formula becomes:
(((200 * 811 * (1 - 0) * .005) + 1605) * 0.1) / 12 = 20.13 MP5 = .252 MP5 per Int
Hey, now we're talking! But in any realistic scenario my gut feeling is the best MP5-equivalent you will ever get per Int (with Inspiration) would rarely if ever be above 0.1. My ballpark guess is .03-.05 would be more typical, and there may be rare cases where that small contribution would tip the balance in choosing a particular piece of gear over another. In conclusion, each point of Int does make a contribution to healing efficiency, but it seems clearly quite small in typical situations and difficult to measure with confidence.