I've done that too, come up short. But usually just because I runout of fabric. I do what be of the other quilters said - piece the border, sew it on and THEN cut. If you pin in on right, you won't have any problem stretching or waviness. Also, if I don't have enough fabric, I have added pieces of the fabric I used in the design, into the border, as if I had planned it that way all along. I did this with a Bargello quilt, and I got a lot of compliments on the border.