Fixed VAT rate is a taxation system that is used worldwide. The Fixed VAT Rate allows you to add a certain percentage of value-added tax. For inclusive rates, the defined percentage of tax is back-calculated from the charged amount.
By default, the tax amount for fixed VAT Rate taxation plug-in rounds upward. PortaBilling rounds the tax amount to the pre-defined number of digits applicable to the customer class currency. For example, if the customer class currency is USD which rounding precision is 0.01 (2 decimals), then 1.204, 1.205, and 1.206 all round up to 1.21.
Also, you can change the default rounding method to apply mathematical rounding in the Stats.Tax_Rounding_Method option on the Configuration server. For example, if mathematical rounding is applied and the customer class currency is USD which rounding precision is 0.01 (2 decimals), then 1.204 rounds to 1.20, 1.205, and 1.206 round to 1.21.