I never enter the number of gallons added when filling unless I don’t fill it full. That sort of addition will add up to errors over time as no fuel flow meter system is perfect. When I fill up I know only one thing for sure, the tank is full, so I set the Garmin to “fill up all tanks” (it may be different text on different version of Garmin Chartplotter but there should be a similar option on all versions). What this does is recalibrate the Garmin every time I fill up leaving only the error in the fuel flow meter (Which is what it is. In my case about 5% in my favor). Any other entry will at a minimum carry the previous error forward which will build up the error over time.