FWIW, I typically filll up about 4 gal, sometimes about 6. Results are pretty consistent as long as I fill up at the same station. Since I have no Scangauge or SuperMID I like to top off frequently so I can get the "feedback" re. how I'm doing.
So I wouldn't worry that the "short" tank threw off the results.
I think the nice mpg is due mostly to your diligence and hard work.
Currently getting +/- 50 mpg in fall weather. EPA is 31/39 so not too shabby. WAI, fuel cutoff switch, full belly pan, smooth wheel covers.
Any errors would show up in the long term calculations anyway. I don't think short fills are bad, especially if someone doesn't drive often and doesn't have a scangauge since there's no other way of checking their FE.