Sorry misread. Normally you fill once the bottle can't fill your pcp tank. So if your max fill on your gun is 3500. You fill your tank to 4500 and use it until it pretty much balances with the gun, example. You connect your pcp and open your valve to fill. Now if the tank doesnt fill up or only fills a little of the tank on the gun. Time to refill your tank.

. So once it goes under the pressure of the max of the gun I normally fill my tank.
 
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Some dive shops will stick your tank in cool water/ice as they fill so you will get pretty much the full 4500. If not some shops are willing to let the tank cool back to room temp after their first fill, and then top it off to the full 4500. It would necessitate that you leave the tank with them for awhile or possibly overnight. You would have to evaluate whether the time/travel is worth it for maybe 3 extra fills on your airgun.