Youre missing a very simple logic

You fill the tank of your car and put the miles into fuelpad
You drive any number of miles, 1, 10, 1000
You fill your car again, and put the miles AND the volume filled into fuelpad

See, fuelpad doesnt care about the 1/3 left in the tank - you drove some distance, and used some fuel, you find out how far you drove by looking at the odometer. You find out how much fuel you used by looking at the pump (because when you started filling there was e.g. 50 litres of EMPTY SPACE in your tank where fuel used to be, and after you filled up again, you know how big that empty space was, because you just filled it with fuel


Technically, you dont need to fill each time, you just have to make a fuelpad entry when the level of fuel in the tank is at the SAME level as it was the last time you made a fuelpad entry

If you had a SUPER ACCURATE fuel gauge that told you exactly when you had a half full tank you could stop the car, enter the miles, fill up, drive til its empty, fill up, drive til its at exactly 50% again and enter into fuelpad

The thing is, most gauges are so inaccurate that the ONLY GOOD POINT OF REFERENCE is when the tank is full (because you know the fuel in the tank is at the same level as a week ago, because you can see it in the filler neck )