Handyandy, I think it is the amount of air energizing the boundary layer on the Notar-type blown slot that determines the lift created. I believe there was an experimental rotor with round blades and a blown slot that produced lift coefficients of nearly 4 (That is a lot Widgeon(tongue in cheek)). The downwash effect alone on a tail boom as spindly as a robbo(are you sure you wouldn't rather save harder and fly a 206?) would be negligible IMO.
Tone - a good point about the autos - when you airtest a Sea King you have to be able to yaw the ac right through 90 degrees in a 70 kt auto to prove the rigging of the TR.