you aren't going far enough to really be accurate. at that level if you had gotten in only .3 gallons more your numbers would be way off what they are now.
you need to get into the 10 gallon or so fill-up range.
Regardless if I let the tank run dry, or run it for 1/4 tank the point is I filled completely, ran it, and filled completely, and then calculated. 18.72 corrected on last tank, versus 20.18 corrected this tank, taking into account the changes made validate the 18.72 tank run.