Measuring for the borders is very important and those measurements do need to be done in the centers of the quilt. Repeat for each direction and for each additional border. Cut the border section and pin it to your quilt starting in the center of each side. Then sew it. You get the distorted corners when you sew lengths of borders to quilt without pinning. The fabric on the bottom (feed dog side) when sewing always take up more fabric than the top (needle side). You prevent this by pinning carefully from the center. If I am doing a long border I divide the quilt and the border in quarters.