there are always a number of ways you can do it. I would probably say something along the lines of "if the actual completion date field is blank, then it's not complete, otherwise it is complete", so... making it up as I go along and not actually testing this calculation: IF(ISBLANK(Actual Completion Date), CONCAT("INCOMPLETE"),CONCAT("COMPLETE")).
But you can pretty much pick your line in the sand. "if the status is X, Y, and Z, it's complete, otherwise it's not complete" for example, would work. And so on.