Originally Posted by
Malfecto
From my experience, being haste forged and using scorch, you go to 6 stacks and then 1 AB and then a scorch or missles proc if you have it. This AB will take you to 92% mana, then the scorch cast or missles cast time will take you back just at 100% mana, so your next AB is 100% of your mastery bonus. If you drop 2 AB before scorch/missles, you will return to 92% and not return to 100% if you do 1 AB scorch, but bounce between 85% and 92%.
Now, the thing to remember is, and this is from my experience also, is that refreshing your bomb, recasting your rune of power, using ice shield, etc, really anything that uses a global, with enough haste AND while on your rune, will be enough time to almost bump you back to 100%. Really any interruption of the AB will do most of the time if you have enough haste, we just use scorch because it is very little mana right now and still does damage.
Now, I don't know if the same holds true if you forge mastery. I'm assuming the cast time of AB is longer, and therefore it should line up about the same, but maybe not if your haste isn't high enough to support the regen needed. I haven't any experience going this route but it would be easy to test.
I have a mod, TellMeWhen, that I have set for an icon to pop when I am at 92% mana. I am at 35% haste raid buffed, and the icon will pop up for a brief second on my first AB cast after 6 stacks, then disappear, and it is easy to want to cast another AB after, but I know if I do I won't regain my 100% mana. Its getting close though.