Rather than using a formula, why not just use common sense?

Example: Assume you are using a 3000 mah battery and you only want to use 80% of its capacity. Then 80% X 3000 = 2400 mah would be the maximum you want to consume.
If I fly my airplane for 5 minutes (the way I normally fly it all the time) and it only consumes 1200 mah as noted on my charger when I recharge the battery, then it is safe to assume that you can fly the airplane for approximately 10 minutes at which time you would consume approximately the intended 2400 mah maximum. Hope this makes sense.

This estimate will most likely change as the batteries become old and loose some of its original capacity.