Fuel Remaining Calculation

In my 1977 Cardinal RG, I find that THE most reliable indication of fuel remaining is the amount shown on the Avidyne IFD550.  This has been verified by repeated physical measurements on the ground of each fuel tank with a calibrated stick, and certainly NOT by the fuel gauges in the aircraft.  However, I do find that when I fill up fuel, the amount I am able to add is about 5% more than I would have expected based on the indication on the IFD550.  What is the procedure that I can use to modify the calculation the IFD550 is using based on fuel flow?

Thank you!