The key thing to remember on working through "air consumption" is that our "air" is pressurized for use, so we have to consider both the volume and the pressure in the unit of measurement. Personally, I like the "bar-cc" unit of measure and it is just the volume of the reservoir and the change in bar that occurs during use (shooting or filling).
You could do it per shot, but I think you'd get a more accurate average value if you used an actual average over ten shots or more. And if the gauge on the gun is not accurate enough, you could use the one on your fill device to see what the difference between the start and finish is.
For example, if you have a 580cc tank starting at 300 bar, and shoot ten times and end up at 250 bar, you would have used 580cc x 25 bar = 14,500 bar cc across all ten shots, or an average of 1,450 per shot. That would be your answer.
You could then use it for approximate calculations of other things, like total shots per fill - if you refill at 150 bar, you would have 580 x 150 = 87,000 bar cc of air to use, and at 1,450 bar-cc per shot you could expect around 60 shots per fill. The answer won't be perfect as there is some non-linearity in air usage once over about 140 bar or so, but it will be close . . .
