One easy method is to take a business card, fold it in half, and hold it against the rear 1/3 of the shoe while you press the shoe flat against the rim and tighten the bolt. Perfect toe-in every time. Be sure the top and bottom of the shoe contacts the rim at the same time and the shoe hits the center of the rim braking surface. Most pads will have convect washers that let you swivel the pad in three dimensions until you get the desired contact angle.