I found out on my last quilt that after measuring the three measurements and cutting my border fabric to that average number that I then needed to fold my border in half and pin, and fold from each end to the middle and pin so that I have four equally spaced marked points. then I pinned them to the quilt in the same measured spots on the quilt. This allows you to ease in any measurement discrepancy in quarters instead of at the end of the sewing on of the border. I did not do this at first and ended up with too much to ease in at the end, even using a walking foot. It was frustrating.
I hope that made sense. If anyone else can explain better that would be great.