The force from the pressure must be enough to overcome the inertia and drive the mass of the nose section the length of the coupler inside the tube to the point of separation, plus a little more for momentum to fully deploy everything. If you consider the nose having to deploy into a wind, or not near apogee, you need a little more push again. Assume that the gas expands and the pressure occurs instantly and uniformly throughout the volume. The pressure exerts an instant force on the forward bulkhead intended for extention. This is the simplest case for a single set of variables and adequate for most ejection systems.
V is the free volume in cubic inches.
Here's an example calculation. That makes a volume of 226 in 3 . The equation can be turned around to find what pressure is produced by a given charge mass. From this relationship of parameters, you can better design the amount of recovery space in your rocket, or customize the amount of powder to successfully deploy the system. Of course, since you're being this careful to determine the correct charge amount, you'll want to verify it by a ground test.