RE: Formula for determining Battery size needed
Theoretically your 3000mAh 40C pack should be able to deliver 120A (3000mA x 40 = 120A)... in practice one should ask a pack for more than ~80%.........so, about 96A max.
The calculation to get 125A goes like this 125A/40 = 3175mAh... but in practice (using the 80% rule) you'd probably need 125A/40 x 1.25 = ~4000mAh
Just to add in some other examples to demonstrate the method:
1000mAh 20c pack....thus rated for 20A... in practice (80% rule again) = 16A max
1500mAh 25C pack... rated for 37.5A.....in practice (80%) = 30A max