Shielding effectiveness in CST MWS

Hello, can anyone give an advice how to measure shielding effectiveness of PEC plate with little holes in it?
Currently, my CST simulation model is PEC plate with holes in 100mm distance from "Plane wave source". on the other side of plateat distance 100mm are located E-field probes. How to calculate the shielding effectiveness?
During the real life shielding effectiveness measurements, normalization are made firstly, when between two antennas no shielding material is placed. Afterwards, the normalization is finished the shielding material is placed between two antennas and measurements are made.
Should I make simulation as measurements are carried in real life?

I don't know CST, so can't help you on the specifics of that.
Are you looking to simulate this for academic purposes, or do you have a specific practical application in mind, such as shielding around a PCB? The reason I ask is that assuming you did your experiment properly, you would be in the far field. But for a practical shielding application you are likely to be in the near field. As such, you need to think carefully about how this would be simulated, and what results you expect to get from a simulation.
I also expect there is an analytical answer to a single hole in an infinity wide conductor. For multiple holes, in the far field, you can use the array factor. But that wont be any help in the near field.
Deborah