Found this over on the 311s.org site and thought it would be nice to post here for easy reference.
It originated from the factory five site: www.factoryfive.com
Here ya go
Caliper Piston Area (in2) and Master Cylinder Bore Diameter (in)
Up to 3.6 use ⅝
3.6 to 4.2 use 7/10
4.2 to 4.5 use ¾
4.5 to 4.7 use 13/16
4.7 to 5.5 use ⅞
5.5 to 5.9 use 15/16
5.9 to 6.3 use 1
6.3 to 7.6 max use 1⅛
To calculate piston area, use the following equation: πr2 (3.14159 x radius squared.)
If using calipers with more than one piston, calculate the area for each piston, and then add the areas together. If the caliper has pistons on both sides of the rotor, disregard one side.