What experimental techniques should one employ during cruise flight to determine correct airspeed to fly for max range in a headwind or tailwind?
An old technique is "Half the Headwind, all the Tailwind".

So if you're flying into a headwind of 20kts, increase your cruise speed by 10kts. If it's a 20kt tailwind, decrease your Cruise Speed by 20kts (but only as far as BestEnduranceSpeed).