If your shooting hand loads, then I would say the sweet spot for the BOSS would be at the bottom of a lake as far from the shore as I could throw it!
Kidding aside, if your using the BOSS as a brake, there are much better ones out there that are more effective and less damaging to the hearing. The BOSS is the loudest thing I have ever shot!
If your set on using it, it takes a little experminting, Shoot where its at for a base line, then screw it in a ways and see if its better or worse. If worse go the other directions, and repeat.
If you do not hand load, it might be a good time to start. Then you can tune the load to the gun, not the gun to the ammo.