what is Exact mathematical calculation of Y-axis(formula), to find the z(f)??
Try this, it will help:
LMGTFY
Sorry, but I could not resist.
To be honest I have to add some further words on qualification of people who perform testing of electrical equipment:
You really have to be competent and have the proper knowledge and also theoretical background for this. Otherwise you will either run into false results (in worst case harming patients) and you are also putting yourself into danger.
Especially leakage current measurements are complex and some of the tests have to be carried out with potentially hazardous voltages and live parts being accessible!
I don't want to be disrespectful, but such questions as the one quoted let me doubt, that the necessary background and competence is present.
It might be a good idea to get the measurements done by professionals, for example from specialized test labs (ideally providing proof of competence, e.g. by ISO 17025 accreditation).