That sounds like a great start, but checking mileage on such a short trip introduces all kinds of potential for error that can really influence the calculation. Getting 400 miles out of a tankful would be a better indicator of what's really going on. I track the mileage on my new Honda Fit, and I can see a difference when I fill up at a station where the car doesn't sit level like my usual station. It's enough to see 28 or 33 if I fill less than 1/4 tank, and I'm pretty sure it's all due to fitting more (or less) in the filler neck than normal.
