Go up one f-stop. Guide number divided by distance equals f-stop. You set the aperature to f-11 for the perfect exposure. If you use metric, adjust accordingly. But on with the lesson.
Guide number divided by distance equals f-stop.
Guide number divided by distance equals f-stop. Guide number divided by distance equals f-stop. Guide number divided by distance equals f-stop. Guide number divided by f-stop equals ideal subject distance. So you fire the picture as he reaches the spot 4 feet above your head.