I have a few of the tanks like nied posted and agree the gauge works well, errs on the side of showing less has than you have. I got them at BJ's and Sam's Club.
I bought one of these on clearance sale at Tractor Supply a few years ago for the 30# and 40# thanks at my cabin: Dometic LPGC10 LP Gas Checker
https://www.amazon.com/dp/B00Q7379X0..._eYerDb71Z0JDV
It works well.