Easiest way to approximate is with energy conservation.
A 50/4500 has 225,000 inchpounds of potential energy (pressure times volume) when filled to 4,500 psi.
The same tank filled to 2009psi has 100,450 inchpounds of potential energy.
For example (not using your numbers)  If after 10 shots your tank pressure drops from 2,009 to 1,979 psi, then you have consumed 1,500 inchpounds of energy (100,450 inchpounds minus [1,979 psi x 50 cubic inches]), or 150 inchpounds per shot.
Take the 225,000 of the full tank (inchpounds of potential pneumatic energy), divide by the energy per shot (150), and you get the approximate number of shots per tank.
Actual numbers will be slightly different due to some conversion of energy to heat and simplification of the process as described above  but the numbers will be quite close.
You can then compare your numbers to those of different size and pressure tanks just by dividing potential energy of the tank (pressure times volume) by the consumed energy per shot.
