So you cannot avoid the need to characterize the instrument. One key calibration task is to determine the effects of stray capacitance and to minimize them. The KP provides a very effective tool for this. The Probe Characterization sub-routine make the probe oscillate at a frequency which is then swept over a range of frequencies. The sample is removed during this procedure.
Stray capacitances can lead to errors of hundreds of millivolts.
Also, note that the KP method is a differential method. It measures a difference in work functions, not absolute values. In order to measure absolute values it is necessary to have one electrode be at a well defined potential. Most KP6000 users measure change in contact potential difference over time, temperature or some other varible. This accuracy ignores, however, the effects of stray or fringe fields, and other, random electrical interference.