Brazil, witholding tax h

Dear experts,

in Brazil, witholding tax has a fixed amount you have to deduce from the calculated amount.

For example:

You have an invoice about R$ 1.500, witholding % is 7,5, then you have to deduce 107,59 (fixed amount established by law), so you would have to withold R$ 4,91.

When I create witholding tax codes I only see how to reduce the base amount, not how to reduce the witholding amount.

Thank you very much!

Prasad