Mark,
In the classic sense, you should see bead and no rib; essentially looking right down the barrels. However, I don’t know too many shooters who do their best shooting this way. Most tend to shoot best when they see a little rib and the most common “sight picture” (hate to use that analogy when talking shotguns), is the “figure 8.” The “figure 8” is when you shoulder the shotgun (assuming you have a mid bead), you see the front bead stacked right on top of the mid bead, making an “8.” This is probably as good a yardstick as any, but the real answer is: It varies from shooter to shooter, from gun to gun, and the type of shooting you do.
Now there’s a statement that doesn’t help you at all, but unfortunately that’s the way it is. Just start with something that gives you a consistent mount and see how you do at your favorite sport (hunting, trap, skeet, sporting etc.). When you see consistency forming, then you have something you can work with. If you’re consistently shooting low, then raise the comb and see more rib. If you’re shooting left (assuming you’re right handed), then add in a little cast to your stock. Eventually you’ll starting really hitting and that’s when you know you’re either there, or very close.
