Buy two tapes. The beauty of this technique is that you're not constantly moving from one side to the other with the same "measuring stick" and "guessing" at the amount of adjustment. You have measurements on both sides (main wing tip to horiz stab tip) at the same time. You can fasten the tapes to the tips of the main wing, hold the ends of the tape in your hands and adjust the position of the horiz stab with ease.