Tax amounts change automatically when you change the amounts to be taxed

The amount in the Tax field will change whenever you change an amount in the line item area or the Freight field, regardless of whether you've changed the tax amount manually. If you change a transaction's information after manually changing its tax amount, be sure to review the amount in the Tax field again to ensure that the amount is still correct.

If, at a later time, you realize that the original tax amount was correct, open the Tax Information window again and click the Recalculate button. The tax amount will be restored to an amount that's based on the tax codes assigned to the transaction.