Hello,

In Odoo, configuring taxes with a percentage of the taxable base (like the 90% you mention for ISLR) isn't directly done within the tax record itself. However, there are a few ways to achieve this result:

1. Use a Formula in the Tax:
   • You can create a tax and, instead of specifying a fixed percentage, use a formula. In the formula, you can multiply the taxable base by the desired percentage (e.g., 0.90 for 90%).
   • To do this, you need to enable developer mode in Odoo. Then, in the tax configuration, you can edit the formula to calculate the tax on the percentage of the taxable base.
2. Create a Custom Tax Rule (requires development):
   • If the previous solution isn't sufficient, you can create a custom tax rule using an Odoo module. This would require development knowledge in Python and the Odoo architecture.
   • In this custom rule, you can define the logic to calculate the tax based on the specific percentage of the taxable base.
3. Adjust the Taxable Base Before the Tax:
   • Another option is to adjust the taxable base before the tax is applied. For example, you could create a custom field on the product or invoice to enter the percentage of the taxable base that should be used. Then, using an automation or a script, calculate the new taxable base before applying the tax.