Hi,
Go to the site for John Flynn quilting frames. He has a whole write up for piecing your back. He explains why you should not necessarily put a seam down the middle of your quilt. Saving fabric is one good reason to use this method. Also it helps when storing the quilt. He works through all the math, so just follow it step by step for your situation.
It can be found here:
add: http://
to: flynnquilt.com/workshop/FreeLessons/
see: Diagonal Pieced Back
Can't put it all together as a link or will lose the message.
Hope this helps.
Pam M