Payment method discount calculate !!Before tax!!

[dangelo]

Hi there!
J: 2.5.11 and VM 2.0.20b.

How can i calculate the payment discount method on the net summary price of an order?

VM2 calculates this:
(Total + VAT) - discount_payment = Final

What I want is: (Total - discount_payment) + VAT = Final!

someone?