If you are in a hurry for the information and need to guess the figure, won't it be the same as the similar winged PA28 -161. It's the wing form that dictates the fuel quantity? The 161, if memory serves, had USg 50 of which USg 48 was useable, so one would estimate USg20 to each step in a typical 161 wing?
Hope that sounds about right and is of some use.