To the right of each coding line on the Coding tab of the Transaction Details window, you will see a tax code component. Tax amounts are worked out automatically for you.
The tax codes that appear in the drop-down list depend on what country you made the purchase in, and on your company taxation policies in that country. If you hover your mouse over the field, help text for the selected code displays.
When you click the drop-down arrow on the right of the tax code field you will see the tax codes that have been made available to you for this transaction.
This tax calculator will also help you split a transaction across more than one coding line and apply the appropriate tax treatment to each.