Well, you can get all fancy with the maths or just go the 'what looks right usually is' way. Personally I would place the mast where it would give you around 8 degrees hang angle and high enough to give good rotor clearance then make blades of suitable length to give good clearance from the l/e of the fin. You might need to increase the downthrust of the engine to allow for the rotor drag well above the c of g of the plane. 6 degrees is always a good starting point.

I have attached an image of the head system I used on my DC roll gyro, its about Simple G sized so you might want to beef it up a bit for a gyro the size you are building. I have used the Simple-G type hub on models up to .25 powered and 36" diameter rotors.

Have fun

Sean