The best way to calculate customs duty depends on the country you're importing goods into and the specific customs regulations in place. However, here are some general steps to follow:

1. Determine the value of the goods: The customs duty you pay is usually based on the value of the goods you're importing, which includes the cost of the product, shipping, and insurance.
2. Determine the applicable tariff rate: Tariff rates are specific to each product and can vary depending on the country of origin. You can usually find the applicable tariff rate on the website of your country's customs authority.
3. Calculate the customs duty: To calculate the customs duty, multiply the value of the goods by the applicable tariff rate. For example, if the value of the goods is $1,000 and the applicable tariff rate is 10%, then the customs duty would be $100.
4. Check for any additional taxes or fees: In addition to customs duty, there may be other taxes or fees that you need to pay, such as value-added tax (VAT) or handling fees. Check with your country's customs authority to see if there are any additional charges.

It's important to note that customs regulations can be complex and vary between countries, so it's always a good idea to check with your country's customs authority or validate it from third party application like simplyduty.com or any other free website or ask US - logistics provider for more detailed information on customs duty.