I think measuring the border has as much to do with it laying flat as whether you tear or cut. Here is my favorite way to cut borders.
Fold your quilt in half lengthwise and lay it on a flat surface. Lay your two side borders along the folded edge. Slip your mat under one end and trim the end at the quilt top edge ..being careful not to trim the top too! Repeat at the other end. You now have two borders the same size. I always pin mine at each end..in the middle..and then in the middle of the halves, adjusting as I go. Sew your seam and then repeat this process for the top and bottom borders.
I love this method as there is no math!