Calibration can be done by adding fuel to an empty tank ( most accurate) or manually moving the float arm on the sending unit. A 2 point calibration would be setting empty and full. This should be fairly accurate for a rectangular tank. 3 point - empty, 1/2, and full. 5 point - empty, 1/4, 1/2, 3/4, full. The more points calibrated, the more accurate the gauge will be across it's range.