u can calculate the values of Kp and Kn by seeing the model file from which u can get Epox and tox and u
just see the model files and for accurate calculations of Kp and Kn look at the spice mathematical models from Rabaey / Taur & ning
But simulation result of spectre has some differenet.
( I think it's a difference of simulation level)
I think 20% under is very good approach.
Please check attached file.
Now I have not exactly decided correct approach between hand calculation and simulation result.
If anyone knows the method, let me know how to do.