Fill up on the same pump at a low flow rate to exactly the same level every time.
If you fill at a fast flow rate then different pumps (and sometimes the same pumps) can shut off at different levels due to fuel back-up in the fuel tank filler neck.
When conducting comparison high speed FE testing in Germany a few years back this is the method we used, even refuelling at the same time of day over the week to try to get the same temperatures. And also refuelling on a cold tank (i.e. the morning after a test run) so that the fuel in the tank had stabilised to ambient temperature.
Fill the tank until the fuel is at a point in the filler neck that you can repeatedly see, and once it has auto shut off then trickle fuel into the filler neck until it reaches that exact point. Rocking the car a bit using your butt can help get air out of the high points in the tank to ensure there are no air pockets
