Your strobe's TTL setting can usually provide decent exposures. It fails, however, when you have a small object on a dark background, or when using a wide-angle lens. In these circumstances you should switch the strobe to manual and calculate the f-stop using the strobe's guide number. The table gives the appropriate f-stop for proper exposure at a given distance. 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.