I use a Tiffen Q-13 color guide instead of a ruler to line up the edges and keep the scan square and lined up. The advantage of using the Q-13 color guide is that it has a gray scale guide on one side. Use the color picker in the scanning software or your photo editing software to set the white point, gray point, and black point for the scan. To adjust for color balance. It helps. But takes more time. I only do that for scans I really care about and album covers that deserve the extra attention because I think the album or cover is awesome and deserves an extra good scan.