Yeah but that's just a number to learn. 400 feet times the difference between temp and dew. You don't have to remember the calculation behind it although when I read it, I found it very interesting.
Likewise, there are a few other numbers that you simply have to learn:
1 mb = 30 feet (actually a little less than that, and a lot less once you go higher up)
ISA = 15 degrees and 1013.2
spiked lines are cold fronts, lines with half-circles are warm fronts
anticlockwise (left) round a low, clockwise round a high on the Northern hemisphere
one-line TAFs: everybody goes flying, two-line TAFs: those with IMC/IR go flying, three-line TAFs: everybody stays on the ground and bitches about the weather.