I liket to set the drag at 25 percent of line test at stike knowing that drag inceases substantially as line goes out. Many people don't realize how much drag increases when the line is half of the spool.
If the drag is 18 lbs when line is full in case of 6/0 reel, the drag increases to 27 lbs when line is half full and to 54 lbs when line is at the bottom of the spool.
When tuna makes circle under the boat, it is time to push lever to the full to shorten the fight, but I don't recommend to touch the drag lever at all passed the strike positon for novice fishermen.