I am a wee bit math-disabled. I cannot remember a number from one second to the next.
Until I saw this method demonstrated, borders gave me fits (always forgetting the number between the measurement and the cutting).
You lay your quilt down and smooth it down nice and flat.
Cut your 2 borders a couple inches longer than you will need -- on the lengthwise grain, if possible and lay then down together, right on top of one another.
Straighten one end of the borders.
Lay that straightened end down flush with one end of the quilt along the center of the quilt (aligning the border fabric along a center line of the quilt). Smooth the border fabric so it is laying nice and flat.
Take a small cutting mat and place it under the OTHER end of the border.
Use a ruler to whack of whatever part of the border lays beyond the end of the edge of the quilt.
VOILA! The border is exactly the same length as the center of the quilt, and you can be sure they are exactly the same length.
AND you don't have to measure with a ruler, tape measure, etc.
Not brilliant, but I haven't screwed up borders since I began using this method, which is saying a lot.
BethMI