Since I've had to correct at least one professor who wrote a book on helicopters on this, I'm not surprised that this misconception still exists.
You cannot use power for determining best range speed. You have to use fuel flow.
Whether you use True Airspeed and correct for wind, or go straight to groundspeed is another matter.
But you cannot use power for the vertical axis - especially in turbine engine helicopters. Turbines become more efficient at higher power settings, and this seriously affects the calculation of range.
Darkhorse is right - if you want to squeeze every last mile, go OEI (and hope you can go as fast as you can on two engines). Better yet, climb high (assuming no wind, or even better a tailwind).