Stretch the string tight and have the helper move the jug inward to the sidewall face till the string barely touches. Watch from the top of the tire as demonstrated in the following pic. If you’re by your self you can move the jug and gently tap the string till the vibrations stop. Then you can go to the tire and tap the string again to make sure that it barely touches the sidewall face. It takes a little longer if you’re by your self, but if I can do it, anyone can.