Dome Light - Need measurements to cut hole in room

[Wgesnerjr]

My Custom Deluxe (i.e. Nothing Custom or Deluxe about it) was born sans an interior dome light. LMC was kind enough to carry the housing, lens and wiring; but neglected to give measurement for those of us not lucky enough to choose a Silverado.

So, could anyone give me some measurements on the hole for the dome light? The housing is curved to match the ceiling, but how far up from the back window does it go? Also I'm not sure how to find dead center from side to side.

Here are some pictures