We can never get this down to a pure perfect science because there are simply too many variables that go into filling your tank.
The only thing you need to worry about is being consistent. If you like the first click, do it every time. If you like to wait until gas splashes on your hand, do it every time. What's important here is to establish a consistent baseline. Whether or not you are techinically getting +/- 1-2 MPG is insignificant in real world application. Unless you are just anal about details that you have no control over...
Trying to make the most of what God has given me.
