How do I track my pressure curve and shot count?

Use a chronograph and shoot the gun from a full fill all the way down, recording velocity every shot.

On an unregulated gun you will see velocity start low, climb to a plateau, and fall off. Pick the band where velocity stays within an acceptable spread (often within 25 fps) and that is your usable shot count. On a regulated gun you will see a flat plateau and then a sharp drop the moment the reservoir falls below the setpoint, which tells you exactly when to refill.