The measurements of tip-to-ground resistance and tip-to-ground potential for a soldering iron are PASS/FAIL measurements and they cannot be calibrated.

Tip-to-ground potential is a measurement of the leak voltage that can be measured from a soldering iron tip.  Resistive and induction heating elements have the potential to create electric current within the soldering iron tip assembly.  The potential is mitigated by proper grounding of the soldering iron tip by design, and proper insulation of the tip from the heating elements.  As the heating elements age, it is possible that the insulative properties may decay over many years of use, which can result in an increase in the leak voltage measurement of a soldering iron tip.  Oxidation of the soldering iron tip assembly can increase the tip-to-ground resistance and thereby decrease the effective grounding of the soldering iron tip.  This abnormally elevated resistance can also contribute to an elevation of the leak voltage measurement of a soldering iron tip.

Calibration is a two-step process of comparing one measurement against another measurement that is a known standard reference, and making an established relation between the two measurements.  The establishment of the relation between the two measurements is typically the act of adjusting the one measurement to equal the standard reference measurement.

In the case of tip-to-ground resistance and tip-to-ground potential, there are no adjustments that can be made to alter the measurement received.  Any FAIL measurement is an indication that the soldering iron has either been damaged and requires repair, or has not been maintained and requires cleaning.

The process of cleaning the soldering iron tip and replacing worn or damaged parts can return the tip-to-ground resistance and tip-to-ground potential back to factory specifications.