Take a look at the Interagency Policy Statement on Restitution. The Q&A has a question that deals with your specific situation, that is, not adjusting the initial rate. Here's one source of the
Policy Statement, go to page 88 of the document (not the pdf) and read all the way through the answer to question 24.
This calculation is complicated and if you're talking about a number of loans, it would probably be worth your while to get someone like Richard Insley to help you with them.