For those who are not familiar, a modern shock absorber uses a valve piston that flows the oil freely in both directions. The ports in the valve are capped with a stack of thin shims when, under pressure, will deflect to open the ports and allow the fluid to pass.

What you have is a situation where the shims are deflecting under a constant distributed load (the fluid pressure coming through the port) but not over the entire 360° of the face (ports are not totally annular). The last part I'm not that concerned with as I think I can take care of it separately. I've thought about using the disc spring formulas but 1) none that I have found show deflection for a distributed load over the face and 2) the shim starts as a flat and bends upward, which is the opposite of the disc spring.