I play around till it just looks and feels right. And I do think it's probably very close to the Golden Ratio (which I've heard of but haven't applied consciously). I take pictures of the quilt as I audition borders; often the camera helps me "see" what I haven't seen. I also stand back as far as I can, and (if I have time) will let my choice "rest" for a day in my sewing room while I peek in several times a day to make sure I'm satisfied. If I'm stumped, I'll send the pictures to quilty friends for feedback. Just kinda trust the ole gut. It's never failed.