Calculating "back thrust".
A bit of physics first, pressure and thrust are different concepts. Pressure is a force applied over a particular area. Thrust is a measure of force.
P=F/A (Where P is pressure, F is force and A is the area.)
=> P x A = F
In this our firearm example, the force we are interested in is the so called "back thrust". By the formula above the back thrust is equal to the pressure times the area it acts on. The area the pressure acts on is the area of the inside read of the case head. (Note Area equals Pi x radius squared ie area varies by the square of the radius => double the radius = 4 times the area)

An example of the above is to consider the 5.56 Nato vs the 458 SOCOM. Both are designed to operate in AR15 family of rifles. Both cartridges use the same locking surfaces. The 5.56 has a max pressure of between 55,000 - 62,000 psi (depending on standard and pressure measuring protocol) wheres the 458 SOCOM has a max pressure of 35,000 psi. The difference in safe max pressure is due inside case head area.