Using air data set to display NM/Gallon, a pilot can make 10% changes in power and quickly determine the benefit or loss after speed stabilizes.  On my Shadin, I did exactly what George did, pulled power back thinking I could extend my range.  My nm/gallon dropped from 6.0 to 5.x resulting in a longer trip and burning more fuel to cover the same piece of dirt/water.  Increasing power got the NM/Gallon back up and higher.   Sounds like a nice app to create now that we can access ARINC 429 data on certified systems.   Such an app would give best altitude, route and power.  Or, just buy a jet and the app is included in a expensive box.