I am afraid I too would take the border off and redo it.
I have always done the three measures and then averaged them for the borders. Recently a friend who LAs for her business told me she ONLY measures the middle. Most of my quilts are pretty close but occasionally I get one side that off a bit. Here is what I do
Determine how long (and like PatriceJ, I prefer length of fabric to width because of the stretchyness -not even sure that is a word - lol - of the woof threads) and cut two identical pieces.
Pin both ends, then the middle, then the middle of that (you get the drift - lol) ... If there is extra fabric anywhere - ease it in carefully (I use lots of pins for that). If you have one edge where a lot that needs to be eased in, you can use the old seamstress trick - run one line of stitching just inside the seam allowance with a loose tension - then like you were going to gather, gently ease in the fabric you need . Then sew.
Good Luck