Note that Solidus supports value-added tax (VAT) and sales tax out of the box. You would only need to create a custom tax calculator in extraordinary circumstances.
By default, Solidus uses a simple tax rate calculator. It multiples against an item price and adjusts for any promotions to determine tax. However, this tax calculator can be changed if you need to develop a more specialized tax calculator for your application.
But in most cases, you should be able to use the Spree::Calculator::DefaultTax
calculator. It is suitable for both sales tax and VAT scenarios.
If you need to change the default tax calculation behavior, see the default tax calculator specs or its implementation.