I didn't have done exactly the performance that you want, but I have an experience may be helps you.
I CST one can make mathematics or logic equations with the obtained results parameters.
for this, use post processing tab -> template based post processing -> select something in upper pop-up ( e.g. 2D and 3D fields) and then select Mix 3D fields (from lower pop-up), So a window will be open in which you can make your own equation like a simple division or summation and so on.