If you have a fuel flow gauge then a useful method I was shown is to divide Fuel Flow (pounds per hour) by Groundspeed (nautical miles per hour) thus giving you Pounds per Nautical Mile. The lower number you can achieve the better.
With so many variables with your aircraft configuration and the atmosphere you should do a little experimenting to find what works best under the current conditions.
Rotor RPM, collective setting, trim, altitude, shutting down a donk or bringing it to idle are some of the options worth considering.
If you don't have a fuel flow gauge then it's probably best to use the charts in the RFM.
cheers,
bb