Hi
At the top of your bead board paneling, typically you would use a "chair rail" moulding that would sit on top of the bead board and a lip would drop down and cover the top 1/4" or so of the panels
I did do a room for a client where I used a simple bull nose parting bead to cap the moulding. This worked fine because it was paint grade and any gaps between the cap and the panel can be chaulked.