Reverse flight - I am afraid that just isn't true - the optimum 'glidespeed' is the one that gives you minimum RoD (and therefore more time in the air) and is usually similar to your minimum power speed in normal flight. Your disc attitude doesn't change much in many helicopters through the speed range because of the horizontal stabiliser.
The optimum speed for range is, as Ramen showed earlier, the best ratio of forward speed to RoD.