Doing a 5 point calibration will provide the most accurate level reading. This is true even if you never set an empty calibration and just use the default cal for the empty point. Of course, if you ever get the tank empty, that would be a good time to set the calibration for that point.
If you start with a 100 gallon tank which is full, burn off 25 gallons as indicated by trip data on your instrument and set the 3/4 level calibration. Burn off another 25 gallons and then set the 1/2 level calibration, and so on...
Correct. Until you set a calibration point, the default value will be retained and used. As you work your way through the 5 points, your level reading will become more accurate.