Rear Spoiler Pitch/angle

[carguy47]

It depends what you want from the device.

In a race situation it provides down force to hold down the rear of the car.  At normal road speeds the effect is minimal on either down force or drag.

The least drag and down-force will be provided by offering the narrowest profile to the air flow.  Best LOOK will be a LITTLE nose down from horizontal IMO.

Agreed. It would also depend on the rake of the car during it's "normal" stance.
Secondly, IF for racing, it would depend on how much droop the car develops at takeoff!