Last Christmas I made my mom a lap quilt.  I used a pillow square and surrounded it with borders and a row of paire points.  It looked lovely in her home, but she never used it there,  I brought her to my home and cleaned out and sold her house......at any rate this was an odd size.  I can't remember the dimensions, but it was too wide for a single piece backing. 

I recalled a method that John Flynn had highlighted on the show Simply Quilts.

He pieced the backing in such a way that  resulted in a large enough piece of fabric with a seam from one corner to another.  Well,  I tried it and it worked.....but I did by gosh, by golly...and then when someone asked how to do it ..I didn't really know...Bev and I searched John Flynn's site and "HEY!"  there is a formula on how to do it.....mathematical formula!   I did say I hate math, right?

Here is the formula

LF=LQ+_LQ(WQ-WF)

               2WF-WQ

Here is a link for John Flynn

http:flynn.com/workshop/FreeLessons/

half way down the page...a story of how it came about  and how to do it.
Work it out on paper a few times...

As you read this I will be at the hospital with my daughter, having her gall bladder out.
She has a high pain threshold and she has been in agony....spent the holiday in misery....won't go to the hospital, then got a call on a cancellation...so with a bit of scrambling , off she goes today.