Your tank centerline should be at the same level as the center of your carburettor barrel.

With the engine upright, that would be about 1"-1.25" above the engine crankshaft centerline. With the engine inverted,1"-1.25" below and with the engine at 90 degrees side mounted, level with the engine crankshaft.

All this with the plane level, of course. Not nose up, like a tail-wheel plane.

Sincerely,