First, Fvvck Janikowski.
It's just a rule of thumb. Kickers each have their own preferences. To be accurate, you look at where the holder is set and add 10 yards. In college, the holder is typically 7 yards behind the LOS. In the pros, you often see it 8 yards behind the LOS. Not sure why, maybe because the NFL guys have better distance and the edge rushers are all fast? I don't have a source for you beyond my own two eyes and two ears. Next time you watch an NFL game, look for it. I started noticing it a few years ago.