This probably won't be much help, but there's really no magic calculation for borders. Often the inner border is narrower than the outer, but I've seen very lovely quilts with the opposite configuration. I think it depends on the fabrics, and the look you are going after. Some fabrics just seem to recede and I use a wider cut than I would have guessed, others seem to overpower & need to be narrower. I use the audition method where I piece the center blocks, then lay them on the uncut border fabric. I keep adjusting until I like the look. Remember to add seam allowance when you cut!