Duty rates originate with countries. In place of using calculators you can also look at individual country websites to review their duty and tariff information. Information on government websites varies. It will be more in-depth than the calculators but you will also have a chance to build a foundation for understanding their specific duty rate policies.
A Google search should yield government pages. Try "duty rate" name of country. A few sample pages below. Make sure the website is the government website. Look for clues like going to the site home page and what the domain address ends with. Information on government websites will be voluminous and might be too difficult to parse through.