Based on your shown areas, I got 1143 psi chamber pressure for equalization, so the same that you got. It looks like this is an estimate though because it looks like you clicked the surfaces and there are chamfers on them, so the actual diameters will be a little larger, but it should be pretty clos...

> Obviously the downside is the spring isn't variable so a different spring would be needed for a given mix.

A threaded boss at the rear can increase or reduce preload on the spring within a certain range, although this will also reduce spring travel.