The issue with the form rule method is that it relies on the User to edit a record to trigger. I would recommend a daily automation to run a table-to-table import that updates that one field with the value of the formula. Also, any record change that would modify the formula value to another value; should trigger a date/time field update in the record. This can then trigger an automation to update the entry value to the value in the formula; which would be more "real time" than a daily table-run. All 3 options running together should keep everything as current as it could possibly be.
