[tsubaki]

I would simply remove all the fuel, remove the sending unit, clean and inspect the bottom of the tank, replace the sending unit with a new one (making sure the float bottoms out correctly), with the tank level (manipulate the winch stand till so) start making notes at "E" and start adding fuel in 5 gallon increments and noting the locations on the gauge.

I've had two people complain to me about their boat getting good gas hours when they fill it up but then once they reach 1/2 tank the motor just eats up the remaining fuel quickly.

The belly tanks are not usually the same dimension/volume per inch of height therefore you would not expect the gauge to read correctly as to the actual fuel volume.