Bill, I've always been told and most installation drawings I have seen show the centerline of the tank lower then the centerline (spray bar) of the carb. In fact all my Magnum and O.S. owners manual give a figure approximately 1/4th to 3/8ths of an inch lower. However, this measurement is dependant upon the size of the engine, so read your owners manuals! Also, it is more important that the engine thrustline align with the thrustline of the design! Don't just go and change this arbitrarily! You need to raise/lower the fuel tank to match the engine at the airframe's designed thrustline, not the other way around.

Mark DeSchane, Mode One, AMA 59157