For a regulated gun you want to refill before you hit the regulator pressure, so in my case using 100 BAR in my .177 rifles, i would fill when i hit 120 BAR on the bottle pressure.
Or that is i would, but dont, CUZ you see i can not be bothered with pressures while i shoot, so my rifle are always leathered to the 12 L bottle and constantly "filling", and so my on rifle bottle / tube are always at the MAX pressure or dropping very slowly over the next many shooting sessions.
When the 12 L dive bottle are around 120 BAR i go to get it filled back up to the 300 BAR that bottle is ( i also have a 200 BAR steel bottle also 12 L in size, but i use that at home for when i tweak on the guns ASO )
As i have big ( well 0.5 L ) bottles and shoot this little caliber, i have a lot of shots in a bottle, should i decide to shoot not tethered some day, but as i generally shoot from a bench, being tethered with a 5 foot fill hose to the big dive bottle, no problem at all, and i can focus on enjoying shooting.
I also have a unregulated rifle ( old FX cyclone ) here you will see when the rifle are running low on pressure as you just do not hit anything anymore, having such a rifle you also soon get a feel for how many magazines you can shoot before you hit the limit and so avoid bad shots due to too low pressure.
So a not regulated rifle, just figure out how many good shots / magazines you have, and if you like me have a bad memory, put a single pellet aside for every magazine you shoot, that way you can count your pellets to know how many magazines you have shot.
But really if you are just plinking, then falling low on pressure dont matter much, just know when it happen its not the gun that broke you are just down on pressure and need to refill.
PS: If you have a 300 BAR main fill bottle, but the tube / bottle on the rifle only are 200 or 250 BAR, then you of course can not shoot tethered to that bottle as it is too much pressure, in that case you do need to fill in a normal way until the pressure in the big tank are down to what your equipment can handle, and then you can " plug in " permanently and just focus on shooting.
If you shoot the smaller calibers .177 or .22, you can set up the rifle to " sub 12 " if i did that on my rifles i could still shoot the 50 m / yards just fine, and on a fill i would get way more than the 4-500 shots there is in a tin of pellets, you dont need to go full power to have fun, the British being forced to shoot 12 foot pound of power ate testament to this.
You can probably also down power a larger caliber rifle, but it is mainly used on the 2 smaller calibers.
Shooting full power and 30 - 40 foot pound of power ( or more with 20 grain slugs ), i shoot 100 M distance just fine, no problem hitting a golf ball, or scare the living hell out of a fly landing on your target papers ( i have yet to hit a fly at the longer distances )
I mainly shoot 75 M though,,,,, with slugs.
