If you want to completely bypass the Kentico tax calculation in Kentico 7, then the best way to return a custom tax amount is by overriding this method in ShoppingCartItemInfoProvider using a Custom Info Provider:
/// Calculates the sum of all taxes which are applied to all shopping cart item units altogether. Result is in site main currency.
/// <param name="item">Shopping cart item</param>
protected override double CalculateTotalTaxInternal(ShoppingCartItemInfo item)
You can get to the Country and State properties via the item parameter, read the associated objects from the database, and pass the information to your tax service: