Hi UJ,
I'm assuming some things about your gun, but if its pretty close to the standard model, you should get the sort of results you expect. I am not a .20 cal fan, and so have little time with that size on the USFT, but with respect to shot count you should be fine. Interestingly, though the larger cal might use a little more air than the same weight .177, this may equal out because most use the 10.3gr .177, and generally, heavier ammo gives more shots at the same fpe than those from lighter pellets.
In general, as you shoot the gun from the correct charge level, velocity should creep up a little, peak, than begin to fall off. If most shots are falling when you start, the charge start pressure was too low, and if shot velocity is a *consistantly a bit low then climbs rapidly, it likely was initially too high.
When you get your gun, just start with the 1450 psi (on the gun's gage) and check the chrono readings .... I normally set the charge limit by using a pressure that, IF exceeded, would begin to drastically slow initial velocity down .... meaning if 1450 gives a small increase in speed for the first 10 shots, you are likely ok, but try 1500psi and see if there is a large drop (more than 20fps) in velocity compared to the 1450 setting. I would set the recharge pressure to just above where there is a fairly sudden trend in velocity fall 0ff. Generally the highest velocity shots will at the point just a bit lower than the apparent middle of the charge settings you establish.
A cold gun will show less pressure than a warm one, so its good to consider it and maybe stick a reptile temp strip on the airtube. Also, the pressure drops in the tube right after filling a bit, generally around 50psig so develop a method to allow for this.
*surprisingly, these guns can often produce many, many very consistent shots of greatly reduced velocity when overcharged, but then of course the velocity suddenly rises once a point in the pressure curve is reached, ruining accuracy.
