Hmm. That's hard to answer. I started by trying to follow the recommendations from vortekz :
Vortekz.com: Vortekz Generators
However I also wanted to follow my roof to glass line evenly for aesthetic reasons. Since its not exactly straight, I had to ad-lib a little bit. I think they do taper from about 15 degrees in the middle of the car to still holding 5 degrees by the outer edges.