Although financing charges are automatically calculated by most accounting software, understanding the method is useful when determining the amount of a late fee to impose. The financing % should be determined depending on an entire month. First, determine the daily percentage rate. For example, if you charge 2 percent each month, increase it by 0.03, which gets you 1/30th of 2. You'll be paying an average of 0.06 percent daily at this rate. Daily rate multiplied by the total amount owed.