Dave, the measurement you are looking for is
How many inches of Draw does it have.
That is how many inches it can lift the fuel to use, and is independant of the fullness of the tank. It is measured from the top of the liquid to the carb. So the middle of the tank may be an inch or 2 below the top of the fuel, dont matter. Just as in a tall tank, you may have draw problems with suction as the top of the fuel goes from above to below the carb, from siphon pressure to draw vacuum at the inlet. If you tune the engine & can run with 6" of draw, the tank middle could be 5" below and the fuel can go from -4.5 to -5.5... but I wouldnt fly it like that
You go inverted and you get a flood out from the 5" of 'Water'Head pressure . Just like the ole days, pressure expressed in feet or inches of height