Did some quick calculations on my rb20 rec-valve,
Valve seat is 1" diameter = 0.785² inches x 14 = 11lbs
Diaphragm is 2" diameter = 3.14² inches x 14 = 44lbs
estimated spring pressure 10 lbs
so when at 14 psi boost, there is 44lbs pushing down from the diaphragm, 11lbs pushing up on the valve face, then add another 10lbs down for the spring
so thats about 54lbs down compared to the 11lbs pushing back up from the 14psi of boost.
To see how that changes at intermediate throttle positions you would need to put another pressure gauge before the butterfly in the throttle body, needless to say as soon the plenum approaches atmospheric pressure only the spring will be holding the valve closed.
Please correct me if my physics is off