Aircraft fin Local deflection angles

Hello everyone,
I am a new ParaView users so apologize me if this seems basic.
I created and meshed an aircraft geometry using Gmsh. I need to find the deflection angles along the fin (theta shown in the attached image) using ParaView.

I used clip tool to split the fin from the aircraft and started by computing the surface Normals at each point and taking atan (normal_z/normal_x) to find these angles, but I am little confused whether the data I’ve saved is ordered in term of (X,Y,Z) or there are any missing steps that I need to consider. I would really appreciate any guidance. (https://aerospaceweb.org/design/waverider/figures/fig09.jpg)