polarization conversion efficiency in CST MWS

I have a question about circular polarization analysis in CST MWS. I would like to simulate a structure in optical regime which converts from linear to circular polarization light. In some papers the authors evaluate the conversion (from linear to circular) efficiency by CST MWS.
Do you think they use the the axial ration calculated by a farfield monitor to obtain it?
Could you hint me a post-processing method or function for it?