constant poi shift need some advive

try the quarter and a few dollar bills

dollar bill = .003
dime = .056
penny = .059
quarter = .066
nickel = .075

Well i am going to take a guess here, yes you can fill much lower and be better off , with lighter pellets the lower the fill, in general , you can fill to 1900,2000 to peak 14 grain pellets in the 900;s,

fill to 2000 to 2200 to peak 18 grain in the low 900’s , the only reason to fill higher is shot count , and the larger the shot count , the more inconsistent, and the more the groups will move around on the target, more at longer ranges.
Now if your shooting 1″ or so groups this will not come into play so much, because even with the group moving because of psi change , the groups might not vary over 1″ to 1 1/4″ at 50 yards or less. but if your shooting 1/4 ” groups you will see the shot group move center,go high left then come back down lower to the right, back to center, then move low right. yours might go a little differant, but this is what my gun does.

So i eliminate all this by dong a short fill , with 20 to 30 shot string.

Back to my guess, you probably have your power wheel up real high and your string is starting out peaked and it falls from there, so at lower psi tank pressure your running to much power wheel and it is making you shots inconsistent, ???

What is your setting, top hat and power wheel,

My setting for 18 grain pellets into the low 900’s are top hat around .090 , power wheel 3.5 , fill pressure 2100

This is all of coarse is you don’t have a mechanical problem, and hey , are you sure your top hat is not loose. or moving.