Let's examine the numbers on that URL Siksukr cited. The WC (water capacity) of the tank shown is 47.6 lb of water, specific gravity 1.0 at room temperature. At 77 F, propane's density is 0.493 (
https://en.wikipedia.org/wiki/Propane). Full of propane, the contents would weigh 47.6 * 0.493/1.0 = 23.47 lb. The 42% of WC regulation would allow for 20.0 lb, which is 85% of propane capacity. The numbers all agree.