This is my current brake set-up

Front = Girling D12R = Piston Diameter = 42.8mm (1.68" ) Twin pot

Rear = Golf GTi Mk2 = Piston Diameter = 36mm (1.41" ) Single pot, sliding carrier

Pedal is a standard mini set-up so ratio is 6:1 I believe
Car is mid engine, about 60% of weight at rear.

I have a brake balance bar, and can change master cylinders if needed to adjust the balance/feel a bit.

But the Maths is beyond me to work out a suitable master cylinder size, at the moment it's 0.81" and 0.625"

What would be a better size?

[Edited on 17-4-18 by ken555]

cant comment on the front/back ratio required. But on a twin piston caliper where the pistons are opposing you only count the area of ONE of the pistons

I have just read that the master cylinders should be between 12 to 14% of the brake area.

Girling Cyl = 0.81 dia = 0.52 area
Willwood Cyl = 0.625 dia = 0.3 area

Front Girling = 1.68 dia = 5.64 area
Rear VW = 1.41 dia = 1.54 area

14 % front = 0.784
14 % rear = 0.215

Looks like the front could do with bigger.