Please don't be upset! I've been piecing backs on purpose for some time now, and it it makes the back so much more interesting. (I'm sure it started when I ran out of fabric!) You don't have to go all crazy with piecing - just a add a stipe or two if the length is too short. As a suggestion, if you do end up adding a stripe, I would suggest cutting the large part of the back into 2 sections -although try not to put the stripe in the exact middle - off center looks a little more artistic. Then add the fabric you need- maybe even several different fabrics that tie into the front, and then add the remaining large piece. It will look like you decided to do that on purpose - not that you ran out of fabric. Good luck!