Have you quilted this yet? If so, VERY CAREFULLY trim away any excess batting along your border edges with enough left over to fill the binding edge to your chosen width. I would suggest folding your backing fabric as far back from the edge as possible and pin really, really well so to not nick the backing fabric when trimming your batting. Once that process is complete, measure your backing fabric even all the way around the quilt. I would still double fold so the raw edge of your backing is tucked into the quilt. Don't know if you would want/need to then do a line of quilting along the inside edge of your 'binding' to give it more of a 'binding' look or not.