I don't measure my borders - I cut the width I need for the borders and make sure it is longer than the quilt. fold the quilt in half, and lay my borders on the middle of the quilt, and carefully, 2,3 or even 4 times smooth it and make sure top edges are even. When I am satisfied it is as even and straight as it can be, I cut to length while it is on top of the quilt, being careful not to cut the quilt. Fold in halves, quarters, fold the quilt sides in half, quarters etc and put pins together at marks, and pin
I have never had one come out crazy.
Jackey in MO