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

I did a test and now I see that the formula text field does not calculate until the record is actually saved.  So now I'm not sure what to suggest.

Just tested with a different user, different file location, same file and did get the error, as you would expect, with the "fakepath" text.  So, as long as QB doesn't change the way that processes I'm going to call this solved!

OK!