I write down the relevant details and hand-calculate until I do the next fillup.
This last weekend I used it for a highway trip in the other car. From previous trips in that one, I know the calibration details. Just set engine size, fuel %, and speed %. Do a fillup routine to reset the tank and drive away.
Now, I'm back in my car. I set the engine size, fuel %, and speed %, and did a fillup routine. I just have to hand-calculate to combine the 2 partial tanks until the next fillup.
Edit: the way to calculate is to add total miles and total gallons, then divide miles by gallons. You can't do an average of the mpg values and get the right answer. Think L/100km or gallons/mile and not miles/gallon.