To calculate the percentage mark-up for a product costing a retailer $2.20 and sold for $3.19, the mark-up can be determined using two methods. The first method shows a mark-up of 45% when calculated based on the wholesale price, derived from the formula (3.19 - 2.20) / 2.20. Alternatively, when calculated against the retail price, the mark-up is approximately 31.03%, using the formula (3.19 - 2.20) / 3.19. It's important to note that different businesses may adopt varying methods for calculating mark-up based on their reporting needs. Understanding these distinctions is crucial for accurate pricing strategies.