I literally made this account just to post this question because I looked above and beyond and couldn't find a definitive answer to what to prioritize when you have multiple procs at the same time for FoF and Brain Freeze (except the now outdated "it is generally preferred to spend FoF procs before following with a FB -> BF -> IL combo). This - I believe, is outdated advice.
There are three distinct scenarios:
1) (Easy scenario?) You are in the middle of casting Frostbolt with no active procs. You finish casting Frostbolt and immediately start casting another. You see a BF proc from your previous Frostbolt and simply follow up your Frostbolt cast with a BF -> IL combo.
2) (Tough one!) You are in the middle of casting Frostbolt - there are no active procs. You finish casting Frostbolt and immediately start casting another. You see both a FoF and a BF proc come up. Now you have two options:
- follow up with BL -> IL and munch the FoF proc, or
- cast IL after Frostbolt, and then follow up with FB -> BF -> IL in the next round. What do you do in this case?
3) (Easy?) You are in the middle of casting Frostbolt - there are no BF procs but your Blizzard or Frozen Orb is churning away which is creating FoF stacks while you cast. At the end of the cast you finish up your FoF procs with ILs (even if that Frostbolt procs BF) and then follow up with Frostbolt (and a BF -> IL combo if the previous one had procced BF).
I think (1) and (3) are easy (and the way it should be done from what I can tell). But (2) is complicated - basically if you know you have a BF proc lined up for next cast along with FoF procs lying on the side, what do you do? Even more complicated is whether you cast Frozen Orb when you have a BF proc knowing that you might need to munch the FoF procs generated by Frostbolt. Does IV/Bloodlust/etc. affect this? WELP - I'M CONFUSED
