I would remove the sender from the tank, hook up the wires, and move the float from top to bottom to verify if the gauge is reading correctly with or without the reducer.
Currently you are assuming that you actually have 3/4 of a tank of gas. That might not be the case.
Carl

1952 5-window - return to "as built" condition | 1950 3100 with a 235 and a T-5 transmission