Yes Arnaud, you can create a new tax code (duplicate an existing one which is nearly the same - f.e 50% Non Déductible - Frais de voiture (Prix Excl.))

Then allocate the correct % deductible

Don't forget to update the label on invoice (and to double check the tax grid with the accountant ;-))

To help you with the partial deductibility of taxes in Belgium. In Belgium there are some taxes that are not fully deductible like the taxes on maintenance of car. There is the 21% VAT that is normally 100% deductible but in this case it is 30% deductible.

To help you with that you can create a group tax that will be composed of the 70% (of 21% = 14,7%) non deductible and the 30% (of 21% = 6.3%) deductible. Therefore you need to first create those taxes separately to add them into the group tax.

When you create those 2 taxes, be careful to get the correct tax account (example: account 411 TVA deductible and 640 taxes on cars and truck). Furthermore you need to specify a tag on the deductible tax. The grid should be, regarding this case, the 59, 63, 82, 85. Nonetheless, you should always ask the accountant of the customer to double check.