Hello! This depends on the method which you used to measure Kelvin probe. If you used double-pass technique, you need to measure some reference just before your sample. And remeasure the reference before each sample (tip degrades rapidly during the measurement). Typically the reference is HOPG. Then you take a difference in the potentials determined for HOPG and your sample and recalcualte the work function of your material from a known work function of HOPG. 

In the case of a single-pass technique the potential which you measured corresponds with a reasonable accuracy to the work function of the material (determined under specific conditions which are not equal to injection of the electron from the material to vacuum). But I personally never worked with it. 

Dear Kaushik 

Reading "Bruker's application note on Kelvin Probe Force Microscopy" could be a very helpful starting point to understand the the theory and the calculations for KPFM measurements.

Hope this is helpful

Good luck!