I think what you did was a great idea. I've done similar things to the back... Tried to make the center of the quilt perfectly centered on the back... My backs are always pieced, and sometimes I have blocks that, if they matched up to the top, they look really cool with the quilting. Anyway..... I might suggest you pin along the borders and make sure they match up on both sides, like when you're matching points on blocks, then give yourself room to quilt within the borders. Don't quilt right up to the edges.