Agree, measure the angle and divide by two.
Also, I like the method of cutting it upside down. You set the ceiling side on the table of the miter saw. The right side piece is cut from the left side of the blade, and vice-versa. Then you don't have to worry about the compound angle, just the angle of the corner. This is a lot simpler for those of us who don't do trimwork that often.
Another tip would be to use two small scrap pieces to dial in the angle, before you start on the really long ones.
Last edited by sanchez; 01-29-2016 at 10:53 AM.