I am not sure, if you put the gauge before the regultaor it will read kind of like this
Full, Full, Full, Full, Full, Full, Full, Full, Full, Empty
The pressure at this point is a function of tank temperature not the level of propane in the tank.
After the regulator, Same result, the pressure here is roughly 1/2 PSI or 11" of Water (The pressure of a collumn of water 11 inches high).
In short, you plan will not work as you hope (Tell you half full for example)
Only three ways to do that I know of
one is weight
one is a float inside the tank (requires modifications to the tank)
One is to pour boiling water over the tank after attaching a LCD strip the color change point is the level of liquid..
You can also "Sound" the tank, some folks are good at it and if you are drawing gas fast enough feel the temp boundry.