How do a set a formula text field to require a unique value?

This sounds like a great way to get around a formula field not being able to be set to be unique, but I'm having some issues with sequencing.  I've got the formula field and I've got the dynamic form rule, but no matter what I seem to use for the trigger the form rule doesn't fire.  It seems like the formula field is executing after the dynamic rule and nothing gets copied over.  If I were to edit that record it comes in correctly since it is already there.

Any way to fire the action after the form rule executes?