No David, I said the cord of the rotor, or actually its width, not span.
With a helicopter, you have ground effect due to the column of air being displaced through the top of the disk to the bottom that will change the tip vortices making for a more efficient hover with less power needed. This will start to occur approximately ½ the rotor diameter.
A rotorcraft in autorotation will not produce that same column of air as a helicopter in hover, and there is simply not enough airflow downward to effect the tip vortices, so no efficiency is gained when close to the ground.
Therefore, we should not measure using the rotor disk for a rotorcraft in autorotation, but we should measure the same as a fixed wing, by wing width.
Let me explain in this manner; lets imagine we take a rotorblade and mount it on the side of a car making it adjustable up and down and flat to the ground, and then go drive the rig down the road at an appropriate speed (pretending the blade will not flap all over hell). Now begin to lower the blade closer to the ground. At about 2.25 times the width of the blade or about 18” for an 8” blade (actually it would be greater number because the blade is subjected to the same airspeed this way, while spinning it is not), you will start to measure a difference in lift. This will vary by speed and angle of attack.
That is why there is no downwash from a rotorcraft in autorotation that will help in ground effect. The disk of the rotors in autorotation has far too little solidity ratio and mounted too high to make a difference in pressure between the ground and disk for any added lift, also the width of the rotor is too small and mounted too high to gain performance from increased pressure between the ground as a fixed-wing would, and the tip vortices are too high from the ground to be influenced to flow out or decrease in size to provide extra performance.