I never cut my borders until the quilt is done. Then I measure the length of the quilt - in two places - and average those measurements. I always put my border on the bottom, quilt on top when I sew. Less chance of stretching that way - at least for me. Seam allowances can vary so much when you sew a quilt together, it's almost crazy to cut a border when you cut out your pieces - it never fits!