Al, all I can tell you is that the pressure would go immediately (within   2 seconds) to 21-24 psi.  Then as the engine warmed up the pressure would drop to a nominal 8 psi or so

It makes sense to me Ed.  Heat the coolant just a little, and it needs to expand just a little.  In order for any of the coolant to flow out past the cap (even just a little coolant), it must reach the pressure rating of the cap.

How much can the coolant heat up in 2 seconds after start?  Just about zero.  The only thing that is heated a little in that time is the rotor housing walls. Maybe that reduces coolant volume.  In any case, if you have any radiator hose in the system, it will expand enough to keep the pressure from increasing that much.  There is some weird science in here somewhere.

Al