I have a rectangular room, all interior cuts. The first wall(north) I had butt ends on both ends. I made an inside cut on the left end of the east piece and a butt end on the right end. I continued in that fashion for the south wall. Now, for the west wall, I cut the left side of the piece as before, fitted perfectly. The question is: My right end has to fit the left butt end of my first (north) board. How do I cut to fit? Thank you.