So I've been looking at askmrrobot's trinket suggestions and I can't help to think that some of the stat values are weird in the "Trinket Info" page.
I began with looking up the MP5 formula and found this (0.001 + SPI*sqrt(INT)*0.003345)*5 = MP5. The 0.003345 is base_regen for a lvl 85. Now from what I've been reading, MP5 and the "5 second rule" was taken out of the game in patch 4.0.1 and that MP5 (now called just mana regen) only happens when you're out of combat and that the only regen in combat comes from the 5% of base mana (In combat regen).
My question is: what does spirit do exactly and how does it affect the mana regen in combat. I've reforged all over the place before with my Disc priest; from 2200 spirit to 3300+will of unbinding spirit. It doesn't seem to make a difference. Thanks for the clarification in advance.
us . battle . net/wow/en/character/blackhand/Xellis/advanced
us . battle . net/wow/en/character/malganis/Altrix/advanced
Those are my two priests if interested.
As disc you have a passive ability called 'meditation' - it allows 50% of your mana regen from spirit to continue in combat.
That answers your question.
The reason why you dont see a 'great' difference is due to the super imba state that disc priests mana conservation is in right now - rapture's percentage regen every 12secs or so means that most people will start reforing out of spirit once they're familiar with all the encounters, and know how to manage all fights.
Never ask "ok, so how much spirit should i have then", as the answer is completely dependant on how you play - i might finish a fight with 50% mana, you might be out of mana.