You can do a replace field contents on your new field with the calculation you want to use. Once you do that the new field will retain that value unless you manually change it or do the replace again.
It will do this on the found set, so make sure you show all records if that is what you want.
Since Replace Field contents can modify very large numbers of records all in one batch operation and cannot be undone, it's a good idea to make a back up copy of your file before trying this tool out for the first time. That way, you can toss the file and revert to your back up if you run the replace and get unexpected results.
It's also a good tool to research in FileMaker help as it gives you a number of different useful options to choose from.
That worked a treat and saved me a couple of hours typing. I did backup the database, always a good way to start the day. Also, I noted the considerable warnings about using this as part of a user-initiated script. I ran the script once, manually, then deleted it.
Thanks for the quick response.
In the nice-to-have department: a simple make-calculated-values-permanent would be nice, so as not to break existing relationships. I thought "Replace with Current Contents" would be the way to go. However, it copies the one value from the current record into all records of the field - so, not.