There's something seriously wrong with that calculator - observe below.
I wouldn't use this method for a large quilt back; I think it's only appropriate when you have a small quilt and are just a tiny bit short on fabric to do traditional horizontal or vertical piecing. As you point out, cutting a large piece diagonally is difficult, and sewing that long diagonal seam without it warping on you is almost impossible.
http://www.multi-patch.com/