I use Bonnie Hunter's suggestion of not using a measuring tape at all (measuring tapes can stretch).
I fold my quilt in half. Along the folded edge, I lay my 2 pieces of pressed border fabric for that side with one clean cut edge against one end of the quilt. Then I smooth out the fabric all the way to the other edge and cut even with the other edge - be careful not to stretch. If the quilt is larger than the surface I am doing this on, I will pin every so often so it doesn't shift. Then fold both the top and the border pieces to mark the quarter points, pin, matching the quarter points and sew. I pin every 6-8" or so (my preference). Press the seam and fold the quilt in the other direction and repeat. It's much easier than trying to calculate measurements and comes out perfect every time. I have made many quilts this way and never have wavy borders to deal with.
Here is a link to her border hints:
http://quiltville.blogspot.com/2005/...der-hints.html