George,
You plot the drag curve vs airspeed first. Then multiply the drag force by the airspeed to get "Power Required"
Essentially this tilts or rotates the plot to the left. Then a line drawn from the ground speed to the tangent of the curve yields the best range. This Article is good http://joeclarksblog.com/?p=4155.  He says the manufacturers leave this stuff out of the POH to avoid litigators.
 In my 4P ops, I think the power is best at LOP max setting at 65%. This is both the manufacturers recommendation and usually the highest practical LOP power without overtemping the EGT/ITT.  I will look at how this compares to this graphical calculation on the next few flights.
Bill