My beelet made these quilts as one of our projects. You do not as any length to compensate for the diagonal seam. You just sew the 40 strips (which are width of fabric) to each other. BUT REMEMBER TO CUT 18 INCHES OFF THE FIRST STRIP YOU SEW WITH. iF don't all the seams will line up with each other. By cutting the first one off it varies the placement of the seams. We also went on to make Quilts of Valor using this method for the wounded souldiers.