For most helicopters, whatever you get at maximum continuous power is going to give you pretty close to maximum range speed.
There is, however, another small issue to consider.
Specific air range (miles per pound of fuel) vs true airspeed will give you a curve that (in theory) should peak - this is the absolute maximum range airspeed. Then the curve will show a decrease in specific air range at a higher airspeed. If you draw a horizontal line at 99% of maximum specific air range, you will interest the curve at two points. If you use the higher of the two airspeeds, you will take a 1% hit from absolute maximum range, but you will arrive more quickly. When flying into a headwind, this will make a significant difference in time enroute.