We have just rolled out an update to the sales invoice screen that will now allow you to override the VAT amount when creating or editing a sales invoice. This is sometimes necessary when trying to arrive at an exact gross amount (as per the discussion here).
Once enabled (see further down 
), the new column for the VAT amount will appear alongside the VAT rate, as follows and can be edited accordingly.
This editable VAT column is not (at this time) switched on by default. If you want to enable it just go into the Account Settings Menu, select “Company Settings” and click the “Advanced Features” tab. You will then see a tick box to switch this on.
VAT Amount Limit
The VAT amount can be set to any value below that which would be deduced from the rate. There is however a 10p limit to how far above the rate you can go. The 10p buffer was really there to cover any rounding differences.
So for example on an invoice of £100 NET with £20 VAT (@ 20%) will allow you to enter a VAT amount of your choice between 1 penny and £20.10.