In regards to the original posters question about filling up when the tank hits 1/4 of a tank, it's 100% fine and will not skew your numbers. Just try to top it off to the same spot each time you fill it up for the most accurate results. For example, if you always fill it to the first time the pump turns off always stop filling at the same time. Any minor variances will be washed out in the long term average anyway.
most "modern" cars the fuel pump sits in a well/bowl to prevent splashing and also would reduce "gunk" getting to the fuel pump and also the pick up is NOT "floating" so I have a hard time with the 1/4 full rule for gunk sake but I try to stay higher for "emergencies"