I always fold the binding into thirds and crease it with my fingers. This tells me what seam allowance to use so it will wrap perfectly. Then I move my needle so the edge of the presser foot rides on the edge of the fabric and the needle hits on the first crease. THEN I measure to see exactly what that seam allowance is so I know how far to stop from the ends to get a perfect miter at the corners. Hope this helps and makes sense.