I tuck the wavy border under the seam until the wave is gone. The tuck is hidden and none the wiser. Much faster then measuring, just sew on the border and not worry about it. I learned this from an episode of the old quilt tv online, the one with Jodi Davis hosted. I can't find it anymore. I do remember it was filmed at a quilt show and the quilter was giving a tutorial how to make a her pattern and as an afterthought said she never measured her borders and showed how she tucked into the seam to make them lay flat. That one thirty second how to has been my lifesaver for borders.