Pricing in credit memos

Hi all,

I have a case in pricing of credit memo request.

When there is a mistake in discount value-For example instead of 10% they only gave to customer 5% .

Now the make a credit memo request with reference to billing document and to give a credit of additional

5% which - is missing,they have to enter a discount of 95 % to calculate from base value for system to calculate in the total net amount the missing 5%.....

How could I reverse the calculation-That the can enter in credit memo 5 % of discount for system to calculate correct amount automatecly?

Is there some solution for this?

Thanks and best rregards

L