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Resistance rates I think is the primary factor in varying damage on QD. Best I've done is 366 Wind Shot on Kirin (which I am assuming is unresisted) and other times it'll hit him for like 43 damage.

Magic Critical Hit rate is a pretty new concept due to it's recent introduction as a given status on new gear. However, I'm sure many BLMs (and even CORs maybe) notice a higher than normal spike damage that goes beyond that of an unresisted spell/QD prior to any announcement of gear holding this bonus stat. This is what I could only describe as: a critical hit spell. The only real downside is it's never mentioned in the chat log aside from unusually high damage which we'd have to take note on ourselves.

Much more testing will be needed until we can figure out the base % of critical spell strikes let alone what attributes affect it other than the new given status itself via Wise Grip & Witch Sash.