In Kentico's Store Configuration, you can set tax based on State and just put the same value in (fully capable in UI with no code, but a little tedious).
Otherwise you can create a custom Provider override and override the calculate tax:
CMS.Ecommerce.ShoppingCartInfoProvider.CalculateTotalItemsTax or
CMS.Ecommerce.ShoppingCartInfoProvider.CalculateTotalTax
Here's how you do it:
https://docs.kentico.com/display/K9/Custom+Info+provider+example