Auto-calculate field?

I have a currency field where I enter an order's gross invoice total.

Is it possible to create another field that automatically calculates a certain percentage of that gross amount?

We have to pay an 8.5% royalty on our orders, so I would like this amount to auto calculate if possible.

If the gross invoice total is entered as $500, I want the custom field to calculate of royalty amount owed, which would be $42.50.

Thanks in advance!