Basic formula for calculating required cable area
I have a problem with equations like 


  ((distance * amps * 0.04) / ((voltage * %drop) / 100 )) * 100) / 100


Too many terms, too many brackets. What if you transcribe it incorrectly? Where do all the terms come from? Do we need all those factors of 100?

I much prefer to build up understanding a step at a time.

The resistance of a length of pure annealed copper wire of cross section \$A mm^2\$ and length \$Lm\$ at room temperature is \$R=0.017\frac{L}{A}\Omega\$.

Note I'm not using strict SI here, I've put the area in square mm, the way the wire in the DIY store is sized. To stay in strict SI, the area would be square metres, and the resistivity factor would be 17n ohms, usually written as 1.7e-8. 

Is 17m ohms the right resistivity to use? At a higher temperature, if the wire was work hardened, if it was slightly impure, if it was slightly undersize, it would be more. 20m ohms might be a better 'worst case' figure to use, and easier to do sums with.

The voltage drop when it's carrying a current of \$I\$ amps is \$V=IR\$.

We usually relate the voltage drop to the supply voltage, 1v lost in 12v is worse than 1v lost in 48v or 240v, so we need to divide by the supply voltage to get the fractional drop, and optionally multiply by 100 to get to percent.

To put it all in one expression, the percentage drop for a system is $$pcdrop = \frac{I\frac{0.02L}{Area}}{V_{supply}}\times100$$Obviously it can be simplified to \$\frac{20IL}{V_{supply}Area}\$, but the first form keeps the correspondence between the factors and the way we derived it. Personally, I prefer to work out resistance, then work out absolute drop, then work out percentage drop, but each to his own.

Generally with high voltage systems (240v), we work out cable size on temperature rise, and then do a voltage drop check as an afterthought. On low voltage systems (12v), voltage drop tends to bite first. It's worth checking that any calculated cable is OK for temperature rise, but it's usually a no-brainer. For instance, if your 10A circuit needs 16mm2 cable based on voltage drop, there will be absolutely no problem with temperature rise, as 16mm cable is rated 100A.