This is wholly dependent on your country of origin and the royalty tax treaty in place with the US. All countries will have a different treaty and different royalty withholding value outlined therein. If you have filled out the tax interview then you should be set to the require percentage for your country.
For non-US citizens, tax reduction will depend on the existence of a tax parity or equivalent arrangement with the United States. But even then, you still need to pay income or similar taxes in your home country. No way around taxes. Even if you get a 0% reduction in your US earnings, your own country will tax your earnings (based on the home country's tax system).