Which master cylinder bore size to use for power disc Kelsey/Hayes

All the m/c diameters worked in our tests.
The difference was in the "feel and control" over the application and release, - not how much effort.
The travel of the pedal, and the drivers control over the brake modulation.

Saying it takes n% more, - more than what, - you have to feel the difference, a number is just a number until your able to compare by feel.