VAT is calculated on a line by line basis and rounded to the nearest penny.
To adjust the calculated VAT amount, please follow these steps:
1. Set "Line amounts are" to "Tax Exclusive"
2. Press the edit symbol
3. Enter the adjusted tax rate
Please note that adjusting VAT amounts is currently only enabled for the UK and only if line amounts are set to "Tax Exclusive".
An alternative way to adjust the tax amount is to split the invoice into 2 separate lines (one with tax and one without tax). Please see Split an invoice by tax amount for details.