RE: Standoff length for DA-50?
Mike,
Use Carbon Fiber Mejlik 21-11 1/2 3 bladed prop and it will be really quit with very good perfromance. You'll even have one more inch extra clearance. Hope this helps. I have tried it and it's really quit in the air. As a rule of thumb higher pitch, lower diameter will give you less noise.
Good Luck,
Reza
Quique's Aircraft Support