If I topped off, I hit 'Full' (that number determined in settings and can be set to 29 gals for a little margin) If I took on 10 gals say, I rotate the knob to add 10 gals. I have found the fuel system in the SC with Skyview to be very accurate and trustworthy (but verify!). If you hit 'Match' when full the quantity will bump to 24 gals, since fuel level is unreadable above 12 gals./side due to sloping fuel tank and sender position.