I use my large ruler with the 45* line to mark my stitching line on one border. I turn over the quilt and match up the border fabric and sew the stitching line while the extra fabric is still there. I check the miter after sewing to make sure it is laying flat. If everything is good, I trim off the extra fabric. I find if you trim off the extra fabric before sewing, the bias edges are going to stretch more. Once the bias edges stretch, it is hard to tell if the stitching line was right or not.