for car or for home?

Run it and check pressure, use gauge. And topup, dont overfill. Dont forget change the cheap valve.

After initial full filled, you can rely on pressure, but if there is leak, just use low side pressure as indication.

Typically you need to weight the gas bottle, else mere experiment / experience on right volume by weight. For car got eyesight, should have similar for house. You diy service / refill?

ah yes sight glass

one more is u can fee the low pressure line chilled

need to find sweetspot

overall cheap to DIY especially if got many units at home

try and error then, try to find sweet spot and stop filling.