This gives you a 70 percent cost-to-retail ratio. For the year, you book $336,000 in net sales. Your cost of goods sold under the retail method is 70 percent of net sales, or $235,200. Your gross profit is $100,800, which is 30 percent of net sales.You can shortcut the calculation of gross profit margin by subtracting your cost-to-retail percentage of 70 percent from 100 percent. While easy enough to calculate, the gross margin number is completely a function of your cost-to-retail ratio.his gives you a 70 percent cost-to-retail ratio. For the year, you book $336,000 in net sales. Your cost of goods sold under the retail method is 70 percent of net sales, or $235,200. Your gross profit is $100,800, which is 30 percent of net sales.