http://www.multi-patch.com/html/fabric_calc.php I usually do the algebra myself but couldn't find my formula so I tried to find it again and make more copies. I found that John Flynn has a calculator to do it for you! Backs pieced on the diagonal are so much easier for frame quilting because you don't get a big lump from the seam rolling on top of itself when you have a vertical seam.
in case the link doesn't work: I just googled Flynn formula for wide quilt backs and I think this was the first choice.