You should count all of your carbs and then subtract the carbs that come from fiber since your body cannot use it for energy. For example, 1/2 cup of oatmeal has 27 g of carbs and 4 grams of fiber, so it really has 23 grams of usable carbs.
All that nonsense about not counting glycerin as a carb because of it's negligable effect on blood sugar levels is b.s. Glycerin is still a carb and is used by your body for energy where as fiber is not.