The system uses standard tax calculation methods used by account software such as Xero and QuickBooks.
Specifically, tax is calculated on each line item and then rounded before being added to the total for tax. This can lead to some unexpected results if you are not aware of this.
For example, if I have a net value of £8,100 and apply tax at 5% I would expect to get a tax amount of £405.00. However, if my quote or invoice consists of more than one item and these items have quantities that have a decimal place (e.g. 22.5 instead of 22) it can cause the tax value to be rounded upwards on each item. When they are added for the tax total it gives £405.01.
Here are examples of this in Zigaflow, QuickBooks and Xero.
Zigaflow
QuickBooks
Xero
Handling Rounding Xero
If the supplier's invoice/bill is rounded differently to Zigaflow and Xero, you can allow for this in Xero by putting the supplier's total in the Total box: