Hello Rob

The idea of the feeling and using of your top lip is the bedsheet.
The top lip reacts on the airflow.

You have to learn and practice the feeling of aaaaaaoooo to change the pitch first.

With the top lip ( bedsheet) and the bottom lip (Milk spout)
That will give you mor and more a good relaxed feeling to play a passive low c
Hope that makes it clearer for you.
Regards
Hajo🎺 Germany 🇩🇪