Interesting exercise, but there will always be a lot of unknowns. Quick example based on the Junkers Ju88:
https://en.wikipedia.org/wiki/Junkers_Ju_88
1340 Hp engines, so that makes 1340/20 = 67 Imp.Gal per hour, multiply by two to get the consumption for both engines: 134 Imp.Gal/hr.
The example has a 1790 km range at 370 km/hr which is 4.8 hrs endurance. 4.8 times 134 should give you 648 Imp.Gals of fuel in the tanks. The Wiki entry actually includes the fuel tank volume as 637 Imp.Gals, which is pretty close to my guesstimate. Good enough for government work.