[lunarhighway]

first of all straigth is likely the easiets thing to do, as you'll have to conform to the curve of the existing bumper. if you angle it forward you'll trap a small buble of air above it so you'll move the overall stagnation point down a little . but you could compensate this by makeing the straight dam longer.

i wouldn't angle it back, exept perhaps in the middle to allow a little more air to go underneath the car in the midle... (some cars use this strategy, because allowing some air underneath the car gives less net drag than sending it all around it)

what i've found as an average on most modern aerodynamic production cars is that the airdam is about 20 cm above the ground... in front of the wheels and wheelwell this sometimes drops to 15-16 cm, these are not strict figures but they' might give you some reference