Best bet IMO is to run a full string over a chrony with your rifle from a known fill pressure to determine the number of shots the gun will give for a given velocity spread and the associated ideal fill pressure. Fill to this pressure and refill as nec. after the prescribed number of shots.
IMO while pressure gauges have their place on guns, they're usually inaccurate and low-res so should only be used as a ballpark guide. Far better to know how many good shots your gun will give and work within this number, IMO
Originally Posted by Tench
If you have the older bayonet type Air Arms filler that has an external thread around the base you can make a screw on cap with a gauge in it that seals on the filler before pressing the end of the valve in to pressurise the gauge that can be fitted after filling and left in place.
Nice idea - hadn't thought of that!