Perhaps I'm not understanding, but it seems that if you have a seam allowance for the binding, that seam allowance would fill a narrow binding. Perhaps you were wanting a wider binding? I usually just trim the top, batting, and binding at the seam allowance width and then wrap the binding around that to the other side. It leaves the binding full. I look forward to more responses so I will know if I've been doing this wrong after all this time. Besides cutting and piecing, binding is my next favorite process in the making of a quilt.