I have very good results with Bonnie Hunter's method and have not "measured" a border since. I fold my pressed quilt top in half the long way, smooth it out, lay the border pieces out on top, smooth those. I pin every so often so they don't shift. Then I cut either end even with the top. I fold the borders into quarters and either press or pin the edge. Do the same with the top. Match the quarter marks, I pin ALOT (every 6" or so) and ease the rest in as you sew. Press, repeat for the other edges.
Some fabrics have more "give" than others. Be careful you aren't stretching as you measure.