Thanks, Jason,
A guy who was into gliding (airplanes without engine), told me that everything above 12 degrees becomes an obstruction, while 12 degrees and lower is a deflector.
I don't know if it also applies to cars, but it turned out to be a nice angle.
Regarding roofspoiler placement:
I think a small roofspoiler further up front equals a bigger spoiler further back.
But it also depends on the speed you drive, the faster you go, the bigger the spoiler must be.
But I am no aerodynamics engineer...
Greetings,
Gerrelt.