At work I'm sometimes in situations where I have to do this with a core drill. I find that it's easy for me to keep the core bit plumb from side to side as I'm looking down on it, but whether or not I need to move the drill toward or away from myself is more difficult to judge. Having an extra set of eyes watching you from the side is a big help. If I have a co-worker eyeball it from beside me, and take a mental note of the sight picture, I generally have little trouble drilling straight down.