Where the heck is the fuel gauge calibration screw???????

The only way I have ever been able to "calibrate" a vehicle fuel gauge is to remove the sending unit, bend the float arm, and reinstall the unit. Repeat as necessary until the desired calibration is achieved.
The sending unit will very likely be in the top of the tank, approximately in the middle. Since the manufacturers usually are not smart enough to build an access panel in the floor, the only means of access is to lower the tank.
Good luck.