I also don't know what the values in that formula represent.
Good question.
The 12.9 (or 10.4) is the resistance constant for the wire. It tells you how many circular mils you need to get a resistance of 1 ohm over 1 foot of length at a particular temperature. Since the resistance is lower at lower temperature, when the wire is cool you need fewer circular mils to keep the resistance down.
Distance is just distance, and you multiply by 2 because in a single phase circuit you have to go there and come back again. With a pole 1000 feet away you have 2000 feet of wire in the circuit.
If you just calculated 10.4 * 2 * distance you would calculate the cross section needed to give your circuit 1 ohm of resistance.
The next values are amps, which is your load and '3% max VD' which is the voltage you are permitted to lose in the resistance of the wire.
3% of 240 is 7.2, so you are permitted to lose 7.2V in the wire. '3% max VD' / amps gives you the maximum allowed resistance to get your maximum allowed voltage drop.
Take your 1 ohm circular mil area and divide my your maximum allowed resistance, and you get your required cross section.
In your equation you have amps/ '3% max VD', the inverse of resistance. But this is simply the mathematical trick of multiplying by the inverse instead of dividing (you know, multiplying by 1/2 instead of dividing by 2; gives the same result).
Hope this helps
Jon