Originally Posted by MTS

And I would cut the last border at least 6" wide (after including the seam allowances on all the previous borders).
That would give you some "working fudge" factor.

You're talking about doing a miter that's about 22" long.

You need to make very sure that you measure exactly when making the border strata - no extra fullness in the fabric. Otherwise, you'll have borders with the topography of the Rocky mountains. ;-)