When I do miters, I measure across the center of the top and mark that length on the border with pins. (I measure from the center out. Then I mark the 1/4" starting and stopping points on the top itself. When I attach the border to the top, I align the outside pins and pin those first. Then I ease the rest of the border evenly on the top. I sew from outside pin to outside pin and repeat with the neighboring side. Then I sew the miter. Never had a wavy one.