Brake piston area calculations are meaningless unless you take pad area and the specific pad friction coefficient/gradient into account.
A large pad brakes very differently compared to a small pad with the same overall pressure applied. That makes it very very difficult to calculate because you need a tonne of data.
The 135i front brembo calipers for instance have huge pads: ~165x74mm
Compare that to the stock z4m pad area: ~112x55mm (total pad height is of course also dependant on what radius the pad grabs).
A bigger pad usually needs less pressure to get the same brakeforce because the pressure/friction ratio usually isnt 100% directly proportional, but to make matter really complicated, these ratio's also change with disc speed....
So in the end imho it all comes down to building it and test it in person on a quite road.
If it doesnt give the expected result, go and change something or build something new
