Never heard it called critical pressure but at least the following are important pressures:
Minimum fill pressure is based on the height of the highest element or pipe in your system (relative to your tank connection point) plus some 'extra' psi to insure it stays flooded. Assuming water it would be height/2.31 + 2 to 5 psi 'flood' pressure.. varies based on experience w/the flood pressure.
Maximum pressure is typically 90% of your relief valve setting. This would be your upper pressure range. Keep in mind elevation differences if necessary.
Those two pressures plus min and max temperature, fluid volume and fluid type is pretty much what you need to size most expansion tank on simple systems.