Next time it will be easier if you don't make the pieced backing symmetrical. That way you don't have to worry about exactly lining up backing and top.
For example, run one or more strips of pieced blocks diagonally through the back (not corner to corner, but edge to edge). This will make the backing piece longer, but you won't have any matching to do. (If you don't want the last blocks cut wonky, you can use a piece of the backing fabric at each end of the strip to "float" the strip in the backing fabric.)
Hope this was a little clearer than mud......