Include / exclude taxes

I have the PRO version.

When I add 20 products from the category Knekki I get the 10 euro off the total price. I figured out that the discount is exclude the taxes (21% BTW in my country). So I guess the calculation is simple 10 euro is 121% and i need to calculate to 100%. And then I have the correct amount.

But with 10 euro it is a little bit frustrating. because when I fill in the exclude amount it shows 9,99 discount and when I add 1 cent of 0.5 cent it fill 10,01. I even tried 0.05 cent!

So my question is, is it possible or a way to fill the include price?

The page I need help with: [log in to see the link]