I had made a king size quilt for my son and his wife and I procrastinated for about 5 years before I decided to try to get the border on it. I was so afraid it would be wavy.
I finally told a dear friend of mine. She happens to be a very experienced quilter/quilt instructor, has written quilting books and has many patterns out.
She said just measure the sides and cut the border to fit that measurement. Same with the ends. It worked like a charm. I didn't need to measure in 3 place and take an average. The quilt laid nice and flat.