Thank you very much for the explanation.
I just found one formula for Zmin and Zmax. But what are Zmin and Zmax ?
The only formula given is Rcal = (Zmin+Zmax)* (1/3)
How did you calculate Zmin = 121k ; Zmax = 509k in your set up of 150k for Rfb and 210k for Rcal. ???
Please explain that.