I had the same issue on my Demon, I tried a few sending units and gauges. Last year I just got fed up and bought a meter match. Well not knowing how far the pickup was from the bottom of the tank. So I ran the car until I thought I was down to @ 4 gal. by milage. So I hooked an elect pump up to the sending unit and pumped it till empty. Now starting with a "empty tank" I put one gal. in the tank " kind of a reserve " and set that as empty. I then added 4 more Gal. in and set that to 1/4 tank. I added 8 more gal. and set that as 3/4 tank. Then I filled the tank @ 3 more gal. and set that as full. Well it's been over a year now and still working fine. It's not perfect but pretty darn close.
Good Luck!