How do u guys calculate miles per gallon?

[GasSavers_fuelmiser]

I have a 15.3 gallon tank according to my owner's manual.

On my last fillup I accumulated 322 miles (counted by odometer) and filled up 13.0 gallons exactly (gas pump receipt). This gives me a 24.769 mpg. Now when I filled up...I did continue to add more until I saw fuel on the top of the opening. I did not spill by the way. Is this how u calculate your mpg?

Now my other question is...does the 15.3 gallon specification include the neck area of my car? The reason I ask this is because i continued to add fuel past the initial automatic turn off by the gas pump. Is there a better way to calculate? Should I use the scangauge?

I know im a bit anal when I ask these questions but hey..where else am i gonna ask these questions? Just want to be as accurate as possible