It's a rather straightforward calculation, is it not?
(X/C) * P = AsFan, where
X = count of measured variable
C = cube size
P = pack size
So in my cube, to calculate my AsFan for removal, we take my removal count (91) for X, divide it by my cube size (460) which is C, and then multiply by my pack size (15), which is P. This gives me a removal AsFan of 2.967, meaning each pack contains, on average, about 3 removal spells.
I'm assuming that I've been doing this right, anyway.. But if I am, it's easy enough to punch into a calculator.
(I'm going to be fairly embarrassed if I'm wrong about this.)