I think you will find that dry firing causes a tank to empty faster than with paint in front of the bolt.
I like the suggestion of the 13.3k as a benchmark. It will give an idea about a usable number of shots while still giving you an indication on the point that your gun stops shooting. If you know the HPR setting this is already known, but I don't have a gauge on any of my guns as I don't care what pressure they run at, within reason.
EDIT: To touch more on the dry fire aspect that IDKFA brought up, spool valves have a finite and preset unit of air for each shot, so those shouldn't be affected with dry firing, poppits would be affected though.