Compute angles for solar control louvres

Hello all,

I am trying to compute the angles between a series of faces - each representing windows and all facing in different directions. I want to use the computed angle to select the faces facing predominately north, east, south, west etc... using equality and a cull pattern of some sort to check whether they fit within an angle/radian domain - so that each frame can be assigned a different louvre treatment based on the domain that it is contained within.

Further more i would like to use the computed values to generate different louvre angles to optimise my shading. I can use surface cp to get the normal direction but how to use this to my advantage and to cull based on parameters- measuring against north angle?

I managed to cull based on angle however i noticed - in returning the angle values the calculation performs based on the shortest angle - rather than around a 360degree circle which seems to make it difficult to separate them as the surfaces facing north and south have similar angles - as do east and west - when ideally i would be able to separate these also.