Hi Pawel,
You can start off using a 200kohm resistor as your Rfb. With the resistance you are measuring (150kohm), you should be able to get the correct reading. We have tried it in bench before, and it can be used to measure 150kohm resistor.
Furthermore, if you know the limits (Zmin and Zmax) of your unknown impedance, you may use the formula that can be found in the application note AN-1252 to define the appropriate RFB. Please refer to Page 5, Choosing an Appropriate Value for Rfb.
Hope this helps.
-Sittie