in this example the price that WooCommerce will calculate to adjust the tax base to the amount we want to invoice the user with taxes included will be 0.99 / 1.21 = 0.818181 euros. The amount of VAT applied to the product will therefore be the remaining (0.171818 euros). You can read more information about tax calculation on this official WooCommerce page (https://github.com/woocommerce/woocommerce/wiki/How-Taxes-Work-in-WooCommerce).