Front - Rear Bias
personally I'd concentrate on the fronts since you only want 20% rear bais but here you go:
Front Force = weight front + total weight * tire friction * height of CG * (1/wheel base)
Rear Force is the same except substitute weight front with weight rear and change the + to a -
The ratio Front/Rear should be used to get the Front Braking Force and Rear Braking Force percentages.
I.E. For a Front/Rear = 3:1 then the use 75% of the Stopping Force for to get the diameter of the MC for the front calipers.