How to add a calculated tax in OpenERP 7
A couple of things:

You can't just add the tax to the invoice total. Invoice totals are stored functional fields that are calculated as the sum of the invoice lines so you would need to adjust the tax on a specific invoice line, or add a line with the taxes.
Taxes are managed via fiscal positions. Create your tax object, then create a fiscal position, then add this as the default for the customer, or just apply it to an invoice on a case by case basis.
I am working with OpenERP 7.
For business requirement, I have to add a tax to the invoice, this tax will be calculated like that : tax1= amount_untaxed * 0,1 % + amount_tax * 0.1 %
A little bit of help will be appreciated .