All this theory is fine, but in the end, you have to make the helicopter do what you need it to do in order to make a successful landing. The airspeed, Nr, theoretical glide angle, etc don't matter at all (within limits, of course), you just use the speed and collective you need to get it to a safe area. Doing textbook autos to a runway do not even begin to make you a proficient or safe helicopter pilot. Get a proficient instructor with a few thousand hours to show you how things work. You can enter an auto from exactly the same point, same altitude, same airspeed, and put the helicopter on almost either end of a runway by varying the airspeed and Nr.