Product-Based Fee/Discount - problem

Hi,
we have this extension "Product-Based Fee / Discount v154.1" installed in our OpenCart 1.5.4.1.

Our problem is that the discounts are not calculated according to the criteria that we use in our country.
Discounts are now calculated on subtotal + taxes.
We would like the discount is calculated on each individual product, without taxes.
This thing is not possible to select it from the module.
How can we do?

Thanks in advance
Greetings

Please contact me at the e-mail address in the extension files, or www.getclearthinking.com/contact, for support with extension issues. I believe we're already discussing this issue, but if not e-mail me and we can talk about it.