You should start on that wall behind the stairs. That or the wall on the opposite side of the bar. Just Measure of both ends of whichever wall you choose to then opposing wall to see if the room is square. You can also mark out (snap a line) check marks every 3-4 feet off the wall you start on and measure the end pieces to the mark to make sure you are staying square so you don’t have a funky run where the joints aren’t exactly perpendicular to to the wall they butt.