That's a loaded question (no pun intended)...

Every gun can shoot every load differently. So I suggest the following.

Take your gun, along with the intended load you want to use and try it at known yardages that you typically shoot ducks at.
Shoot it at a large target (say 2' X 2') and count the pellets in your kill zone.
Try your different tubes, and try different loads, until you get the results your looking for.