As Draigflag say's whatever you find best is the best for you.
I personally use the Odometer reading, I just write it down on the fuel receipt ready for input later, which may be days or weeks, but as long as I have the receipt with the Odometer reading on it, i'm fine.
I do reset the trip meter on each fuel up as well, as I like to keep an eye on both the trip figure and the miles to empty figure, this gives me a good idea of how i'm doing during the current tank.
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