Your batting will dictate the farthest apart you need to secure your quilt. I don't know of any batting that will allow the distance you are considering. even an "X" thru a 15 inch square is probably going to be an issue. Maybe quilt a straight line in the ditch along your sashing then maybe vertical and horizontal lines about 4" in . An all over design would also be good, like swirls, loops or a meander.