How to quantify "far-field" property in my HFSS mo

Standardly, HFSS has the function of calculating RCS for an antenna or 3D object scattering. But my model is actually not exactly a 3D problem. I use periodic boundaries in the z-direction of my computational domain (surrounded by PMLs in x-y plane). I'd like to calculate the "RCS" for the "far-field" in some plane perpendicular to the z axis. How can I manage this?

Thanks for any reply!

RCS, TMz = |Escattered| / |E incident| ?
I think. Not sure. It is in chapter one of the Mittra book on computational methods for Electromagnetics. Actually, it should be in the help file.