Ditto to much of the above.
Remember that the mitre is a 45° and thus, why you need additional beyond the length of the main part of the quilt.
Being that you are creating such a wide border, perhaps draw it out to scale on some graph paper, to calculate the amount you will need. (and as said allow a little extra for the "fudge" factor!)
I would join all the borders and do one large mitre ... you'd have a smoother, straight seam.
Whereas if you add each border and mitre them separately, chances are they won't be continuous, no matter how hard you try ... and then you lose some of the beauty of the mitre!
Yes, JIMHO!
