Smurf,
DO NOT drop the tank. The senders are capacitance types, IIRC. Try this link to calibrate the gauge...
https://www.youtube.com/watch?v=16-i8zBLqgA
Calibrate for Empty (or as empty you want to be when the gauge reads 'E'). You don't really want to be empty, ever. Then fill up and calibrate for Full. For me, empty is about 5 gallons.
The shape of these tanks don't help, either. Due to the sloped bottom, they 'empty' (per the gauge) quick.