Originally Posted by MTS

If you're going to add borders, I suggest you cut down what you have now for backing by at least few inches before adding additional fabric.

You don't want that seam anywhere near the actual edges of your quilt where your binding would go.

Or you could just add a piece on one side, say 8"-10" wide, shifting the whole thing over. Do the same thing for the length.
Don't even have to try to center it. That's probably the easiest.

Pieced backings are great. I do all mine that way.

Just keep that long seam a good way away from the parallel edges.