OK, so plan B. I guess that you have a hard coded formula field with the calculated rate which depends on the location.
I suggest setting up an Action to be triggered when the record is marked as Paid. This would copy the calculated amount into a plain numeric field which of course would be frozen. An Action will be more reliable than a form rule and also will work when multiple records are updated by say Grid Edit.
Now, the issue you will run into that on the surface it will appear that a Action can only Add a record or edit child records and what you want to do is to act on the record itself.
The Technique, which is perfectly fine to use is to set up a Report Link field which will define the set of records to be edited to be the record itself. So you configure on the left side of the Report Link field properties to be the [Record ID#] field and then navigate on the right side to the app and the same table and the same [Record ID#] field.
Note that the Action will operate under the Permissions authorities of who created the Action, so you may want to consider setting up a userid owned by the IT department to be used for Actions and also any Sync Table connections.
