Idahoguy -- It is rather straight forward. PJs vary in throw distance from the screen (the distance between the front of the lens and the screen), and that distance depends on the size of the screen.
For a 16:9 (1.78:1) 100" diagonal screen, this throw distance can be anywhere from 10' to 26', depending on the projector model. So specifying where the outlet should be is a real crap shoot, if you don't have any idea of what PJ you will/might be getting.
In your case (with a 17' long room) your best guess would be to put the outlet somewhere between the 12' and the 17' distance from your proposed screen wall. However, you still could be 2' to 3' from the eventual location of the PJ. And with some PJ models, you could actually mount the PJ on the back wall.
This is the reason for Jim McC's post. The bottom line is that you just don't know where the PJ will go until you decide on the one you will be getting and what size screen you will be using.
PS -- For most PJs, all of the cables come out of the back of the unit. So the cable access should be behind the PJ mounting.