I would measure my line starting from the hall and carry that line out into the livingroom area and then measure to the back wall to see how straight the hall is to the outside wall. If it is straight then use that line, if it is not then you can tweak it until it is where you want it.
Thank you that helps a lot! Would 3-3/4″ wide boards on either side of the walls in the hallway be to narrow? In other words, I’d have 3 full boards in the middle then on either side of those 3 boards I’d have 3-3/4″ boards. Would that look tacky or be ok?
Which direction should I start from? The bathroom wall or the wall with the closet and bedroom?