Radius Driven by Angle of Vectors

I am trying to solve a problem for a surface treatment. I have a series of panels that I want to be perforated. The perforation radii should be driven by the difference of the surfaces normal to a line representing a sun angle. So, in theory, the more the surface faces the sun, the smaller the perforations, and the farther from facing the sun, the larger the perforations. 

I just can't get the operation to work. I have tried measuring angles, dot product, etc. but I can't get it to work correctly. I may be on the right track, but I may not be doing it in the right order. 

Any help would be appreciated!