For mouth inhale I suggest 100% closed. Even if fully closed it doesn’t seal the air entrance.
If you are not a lung inhale vaper, you’ll notice that GM isn’t a native mouth inhale atty.
Even with a different drip and the airholes closed it remains a little bit more “airy” than needed.
You might have a defective one or maybe you removed the airflow adjustment lever and did not reposition it correctly. When I close my airflow completely (100%), there is no airflow at all as expected.