I am amending the default 'Purchase Order' screen to allow the tax code to be changed for each line item. However I can't see where the 'Total Amount' is calculated. After entering the line items and changing the 'Amount' to reflect the new tax code I hit 'Update' and the 'Amount' is reset. Then when I 'Save' the 'Total Amount' is updated with the correct amount after the tax code change.
Any pointers would be appreciated.