Well according to the Air Arms user manual, the fill pressure should be 190 bar. Mind you, people do say to vary that pressure downwards to see if it helps the power curve. That should give you about 40 consistent shots, maybe a few more. As for the lowest pressure, fill it once you hit about 100-110 bar. Get as low as 60 and it'll start venting. If it does, cock the rifle before filling.

I'm too lazy for all that jazz, I just fill to 190 and be done with it .