You might have better luck if you left the white background whole (square), instead of trying to sew on and off biases. I've done several double wedding quilts, and always appliqued the rings directly on to a square of background fabric. This way, there's no stretching on the bias of the background fabric to mess up the finished dimension. You could even make the background squares a little big so you could size them properly for line-up with the next one. Good luck - It should be a fun project, not a chore. I hope this helps. Carol