We're really looking at curvature so an angle would be subject to a location along that curve.
I'll try 'n put up some images to compliment the 'Template'
Here is a plan-view of the AST Part-C gen-II.It is based upon a the trailing portion of a 3.92:symmetrical wing section.This contour has been used successfully on some of the lowest drag vehicles tested and is my current 'buy' recommendation for modding.
Here (in the center image) is the actual wing section I'm recommending.It is the aft portion,adjusted to the body width of your car which you'd use.
