I spent some time today proving to myself that you do use geometry after grade school in order to get my oct-box for a future ceiling fan perfectly centered in the room and in the center of one of the tin tiles.
Came out well and just as anally retentive as it sounds.
It occurred to me that I had pulled 12/2 through the joists for the future ceiling fan.
Thankfully, I thought about this before I closed up the ceiling. I happened to have some 12/3 on hand and it was easy enough to use the 12/2 to pull the 12/3.
I'm super pleased that I thought about it before it was a major PITA to change. For once.