I agree that two wheels with wheel pants just "look better".
With that said there will probably be a trike design in my future, as a semi scale stunter.
I agree with Jim, that Bob Whitley's 15° is probably a good starting location for the main gear. The negative AOA controlled by the nose gear length is both necessary and potentially problematic. Too much negative and it will jump off when up is applied. The answer here is to have a removable gear to play with. This can be easily done with a mounting similar to a removable wing gear, except with the wire running fore and aft in the nose, below the tank.
I have looked at many trike designs, including those from C. Mackey, Jack Sheeks, and Don Hutchinson. They seem to vary quite a bit with main gear location. Particularly where it exits the wing. Don's all work very well.
One other question I have is.. Is there an ideal distance between the mains and the nose gear.. or does it really matter? I would guess that as long as the wheel base is not too short, almost anything goes???