And thinking ahead...
The packs I am using are ThunderPower 5S 70C 1800mAh (comprised of one 2S pack and one 3S pack connected in series) and Genesis Power 5S 65C 1800mAh (configured the same).
Then, if the drive in my plane draws about 50A, then the measured cell IR I'm looking for is 4.3 (in the F.O.M. calculator, when I set the capacity to 1800 and the IR to 4.3 the Max Current comes out to 50).
Also, during a flight I draw only about 1100mAh from the pack (but it's right at 50A the duration).
From what I gather, 4.3 is extremely low. But if I'm drawing only 50A from an 1800mAh pack that's less than 30C anyway.