I am using COMSOL's Wave Optics module to simulate a simple beam steering metasurface comprised of silicon posts - very similar to Metasurface Beam Deflector (ID: 115971) in their Application Gallery. In the electric field profile, I can clearly see an angle of deflection on the xz-plane compared to incident light parallel to the z-axis. I have propagation constants and S-parameters from my simulation results.

For postprocessing, I want to find the expected deflection angle. How would I accurately determine this? I have been using Euler's formula with the real and complex components of my S21 value, but the results have been inconsistent. Perhaps I have the orientation wrong? Attached are some screenshots and one of my simulation files.

I greatly appreciate any insight you may have.