Thanks for the link from "full tank vs. partial fill ups..." to here. I'll bump this year old thread.
FWIW, and it ain't much, I have found over the years that filling up the tank when it is 3/4 or more full almost always gives me a better FE figure.
This seems to be true now and goes back to the days before auto shutoff pump nozzles and when you could see the gas in the tank or filler neck after carefully listening for that sound of fuel coming up the neck. It was easier to make sure you filled to the same level every time. Same thing after the auto shutoff nozzle. After it clicks off, you top it to the same level.
Unless of course if you were going to park it in the sun for a few hours afterward.
This is purely anecdotal as they say. All those pocket note books I carefully kept over the years are gone.
I have no explanation for this nor will I offer one. Just sayin'. I would see up to a 10% improvement when only burning a few gallons out of the tank.
IMHO, burning it down to 1/4 or less is more accurate.