Very useful post @kha, thanks! By the way, it is required to have installed module "account_tax_python" to be able to find the Phyton Code option as a tax computation.

Hello Kevin. 

Thank you for your contribution. I wanted to add some more information on that atter

1. Do not forget to add "* quantity" at the end of the expression so that it applies to sales with more than one item sold. See below
2. Furthermore, there are two possibilities : Tax included prices and tax excluded prices.

Here are the expression for the two : 

Tax excluded prices

• result = (price_unit - product.standard_price) * 0.21 * quantity

Tax included prices

• result = ((price_unit - standard_price) -((price_unit - standard_price) / 1.21)) * quantity
• In this case, you would need to check the box "included in price" in the "advanced options" tab of the tax

Why don't I have "python code" when I open the drop-down menu ? 

Hey, It s working well on V16 but on the V17 i have this message ovr and over again: "Erreur : : "name 'standard_price' is not defined" while evaluating

'result = (price_unit - product.standard_price)*0.21'"

@juba, it's not working "Erreur : <class 'NameError'>: "name 'standard_price' is not defined" while evaluating
'result = (price_unit - standard_price)*0.085*quantity'"

Hello, 

Your salution is based on a "fixed cost", it is not real. The same item can be bought at 1$, and at 2$ from another provider or at any other time ( prices go up and down ). 

To implement a real taxes on margin you have to make use of serial numbers, and identify every item as unique, with a unique serial number, cost and sale price. 

This is much more complex than this.