The best RANGE speed is simply the power setting which gives you the best MPG displayed on a GPS which is linked to a fuel flowmeter.
Wind will affect the absolute value but you are only looking for a peak in the data.
The power setting will certainly be peak EGT or slightly LOP.
It will depend on weight, loading (and CofG fore/aft), and altitude, and temperature.
I've done extensive tests on this on the TB20.
A lower rpm (2200) is about 10% better than 2400.
Peak EGT is 10% better than 100F ROP and about 20-30% better than flying full rich. Going 25F LOP is only 1% better than peak EGT - within measurement noise really.
Weight seems to make no measurable difference - most likely because there is no way to increase weight without loading the back seats, which moves the CofG aft which reduces the elevator AoA which reduces the elevator drag.... just enough to compensate. I always load any junk in the boot rather than the back seat.
Altitude seems to make no measurable difference - beyond FL100. This suprised me. This has some theoretical support, but I don't understand it. Once wide open throttle (~FL100 say) any additional climb reduces the engine power, which makes the friction and pumping losses greater as a %, but you get a gain due to thinner air. You also get a gain due to reduced exhaust back pressure.
Temperature? Not sure which way this works. But if you leave the plane out, with full tanks, in -10C, and top off the tanks before flight, you are carrying 4% more fuel mass (i.e. energy) than if you depart with the fuel tanks heat-soaked to +30C
But your turbine fuel flowmeter measures volume, not mass...
Got to make sure the plane flies straight i.e. ball in the middle, wings level, ailerons equal (so if solo, run down the LH tank by the appropriate amount). I know of pilots who say that if you have a quartering tailwind you need to use the rudder to fly straight