The length of the border only depends on the length and width of the room and not on its height, but I'm guessing that the door is 2'6" wide and will not require a border. So the perimeter of the room is 2(length+width)=2(12'9"+8'1")=2*20'10"=41'8" less 2'6"=39'2" is the length of the border.