The easiest and most accurate way to measure how much gas you have left is with a scale. I have a digital luggage scale. You just lift the bottle up with it, note the weight and subtract the tare weight of the bottle. It's easy, and very accurate.
That's what we do when we come home, but with the gas bottles in a cabinet on the van, it's a pain in the backside to turn the fridge off, unhook the bottle, take it out, weigh it, then put everything back together again, bleed the line & get the fridge running again
Incidentally, when we get the bottles refilled, we've found a bloke:
https://www.everythingforgas.com/contact-us/, who refills cylinders by the actual amount of gas they take! Weighs them when you bring them in, refills, then weighs them out again & charges by the actual amount they took at $2.80/kg, not just a fixed price