Hi!
When I create a new invoice and want to add a taxposition, I get a form where I can choose the taxtype but must manually insert the netto price of the bill there.
I is not automatically read and inserted from within the invoice form.
The tax lines are computed automatically when taxes are registered on the invoice line. This can be done automatically when selecting a product if this product has taxes defined (via accounting category).
But you can also add manually a tax line on the invoice to adjust special cases.