I would have them measure everything in front of the screen. If they are able to do that area with very little waste, skip the area behind the screen. If you end up with a significant amount of waste, ask them to do the area behind the screen, or you could simply throw some scraps back there.
If your stage has the same carpet as the rest of the room, you may end up with very little waste. The carpet on my stage is a different color than the rest of the room (solid black), so I'll have quite a bit of waste from covering such a small area.