I've seen convex buttons from the Standard Pneumatic factory and
flat buttons as well. Action makers changed mechanism configurations
yearly, it seems.
I don't think the button shape makes much difference. One eighth-inch
clearance seems a bit excessive. Some small clearance is needed, but
I don't think the measurement is critical, unless there is too much.
I suspect, even when not activated, the pouch gently lays against the
button when the system is under vacuum no matter what the clearance is.
The a major factor in getting good pouch power, quickness, and throw is
the right sealant.