For a Constant Attitude EOL - ie there is no flare - we used to teach that you wait until the green turns into grass (between 20 and 30' agl) and make a short, sharp check up with the lever to begin to reduce the RoD, then cushion the touchdown using the rest of the Nr. It is technique that works well in a reasonably high inertia rotor system but not something I would like to try in a robbie.
I did fly with a colleague (RN) at Shawbury who delighted in checking your bottle by leaving all the lever movement until about 6 inches from the ground and then pulling to 16 degrees of pitch in about half a second. The aircraft just shuddered a bit and plopped onto the ground quite gently but it wasn't a technique you could afford to get wrong
JFR where are you?
Denlopviper - make sure you pay attention in groundschool - it is the rate or air movement through the rotors that produces the autorotative force to keep the blades turning - you can autorotate and any speed from zero to Vmax and the only thing that will change is the RoD. Most helicopters have an optimum auto speed between 50 and 70 knots where RoD will be least - above and below that speed the RoD will be higher.