Can I set up a formula field so that a user can overwrite the information when in edit mode?

Here is my specific situation...
I have a stipend field and a benefits field.  Both are currency.
Once I enter the stipend amount, I want the benefits field to be auto-populated as 25% of the stipend.  This is our estimate.  
Once the stipend has been processed and paid, I will know the actual benefits dollar amount.
I would like to overwrite the auto-populated benefit amount with the exact benefits amount in the same field.