From BCA rules.
3.2 RACKING THE BALLS
When racking the balls a triangle must be used, and the apex ball is to be spotted on the foot spot. All the balls must be lined up behind the apex ball and pressed together so that they all have contact with each other.
I would take this as the head ball is to be racked on the spot. NOT above it, NOT behind it. Even though there is a divot from others racking incorrectly, you can always get a rack to whereas one side of the head ball is touching the second ball. I then break towards that side. (Driving into the frozen spot). Heck, if worse comes to worse, let the guy rack his own and then inspect the rack.