I lay my quilt out, lay the border fabric on top of the quilt in the middle lengthwise, cut it to fit. Same thing the other way. I usually do the sides first, then top and bottom. I do not use a measuring tape and I do not average - I just use the center measurement. I fold the border into 1/4ths and press a small crease near the edge. I match up with same on the quilt and then pin in between. I was reading on Quiltville that the fabric or measuring tape can stretch. If you smoothe out the quilt, and also smooth the border fabric well, it lays nice and flat for cutting. I never have a problem with wavy borders now.