Go to your propane filling station and ask the tech to explain the WC, TW and date # s on your bottles collar.
wikipedia has a fine description of the properties of propane and its safe handling.
For instance, a WC of 72 is the water capacity of the bottle expressed in pounds, so correllated to propane 80% fill level at propanes 4.24# / gallon weight, that equals 30# propane. On a temperature compensated meter it works out to about 7.3 gal. propane in 30 # of propane, 4.7 for 20#, 23.56gal. in a 100# bottle, and so on.
Know the math and properties to be certain you get safe filling and your moneys worth, and that the filling tech does also.