Because multi-rotors expend most of their energy maintaining altitude, the best range speed (as opposed to max endurance) is max throttle, esp if flying in a headwind, as the less time spent in the headwind, the better.
The only real factor/variable that comes into play is additional wind resistance due to higher speed, (which increases as a square of the speed...like, go 2 times as fast and wind resistance increases by a factor of 4). But that doesn't much come into play at Phantom speeds, and in any case, the additional power needed by the Phantom to maintain a faster speed is much less than the additional power needed to fly for the additional time at the slower speed.
All the other variables, like temperature and air pressure (and altitude) and such, make a (small) difference, but only if a person can choose the day/time they will fly.
The only technique for getting more range (using less battery) would be to descend at a rate so the multi-rotor reaches the ground at the same time it reaches the landing zone, so you're not spending time descending from cruise altitude right over the landing zone.