If you're always casting T1 after B4, then the math is the same as always casting T1 before B4, and the piety stays the same. You're looking at:
B4 (no tick) > F3 > F1 + 4xF4 > B3 > T1
vs
T1 (no tick) > F3 > F1 + 4xF4 > B3 > B4
So your formula is for all Thunder damage, assuming recast of T1 if you don't get a proc?
I guess you could use that to compare whether or not it's worth casting any Thunder at all if you get a quick MP tick in UI, but it doesn't take that much math to get the answer. The entire rotation without Thunder is around 150 PPS (using your 2.2s GCD), so adding thunder will be a dps gain 99% of the time.
