It is enevitible that when capturing a backwall made from drywall, that edge will not be straight. It will usually have a jagged edge where it has been screwed to the studs.
Adding an offset by using a scribeline action will reduce the time installation crews spend on site.
Add an Offset with Taper
Click Draw > Scribeline
Select Offset with Taper as the Mode if it is not already selected.
Enter the Max Depth > OK
The amount the line would be offset inward from the wall.
3/8″ is usually the most that will be used.
Enter the Taper Length > OK
The distance the line will offset will begin.
Click the first edge that will be offset.
Click the second edge that the first connects with.
Click Done
The result will be an edge on the first line that tapers in the distance entered for Taper Length and a strait edge that a saw can quickly cut.