Make sure your sewing machine tension is not too tight.
If your tension is too tight, there will be many, many small 'ruffles' at the sewing line. Each little ruffle is extra length/fullness on the border. I have had to re-do more than one border because of that little bit of a 'ruffle', and have taken up to 2" off a border after re-do of the border.
Also, when you hear someone say 'press your fabric', listen. Ironing will stretch fabric. As always, I learned that the hard way.