credito memos with automatically amount determination

I would like to generate a credit memo referred to an inoivce.
Suppose this case:
Invoice , alredad sent to the customer
Material 1 -> Gross Price 100 $
Discount                    15%
Net Amount in invoice  85$

I would like to generate a credit memo - with reference to the invoice, but i would like that the user will  indice just
post only the right  % of discount, without to do calcuations.
Suppose that the user will post the right % of discount , that is 40%.
So the credit memo must be generated for the difference between 85 $ and 60$ =25$.
Is the a standard way to do it?