I have used this with 60 lb freon tanks. Take a can of carb cleaner and mist the side of the tank. Using an infrared thermometer measure temp of cylinder top to bottom. Where the temp goes up there's your level. With practice you can find the level within 1/2 inch of where the fluid level is. As the carb cleaner evaporates, the tank will get cold. The liquid lp will not allow the tank sides to cool as much.
Dave and Laura & two cats
02 Discovery with Accord toad
retired auto rv tech and teacher, wife rt nurse