To add a bit of dialogue, I find that a bit more drop [think seeing a bit less rib] is desireable when snap shooting, as in field shooting compared to any of the target games where the gun is typically premounted. I have the thot it is because I tend to have my head more erect in the field, bringing the gun to face & into the target so to speak rather than going through the antics of a down the line[trap] or target shooter where the mount is some sort of drill, inclusive of at least a momentary glance at the beads to check for 'just right', no cant, 'hold point', etc. before calling for the target. I am of the opinion that target & field stocks and their respective 'sight pictures' are generally not the same animal, nor should they be. Trap guns are purposely made to shoot high where the target is as Jimmy says normally rising. Some are made to extremes. OTOH, shooting birds like chucker that are almost always falling targets, blasting off some cliff's edge or rise in a downward direction would tend to be shot best by a gun that if pre-mounted might have you looking at the back of the receiver.

For flairing incomers &/or direct overhead shots the bird, for me, is never seen at the moment of firing because I will have covered it entirely with the muzzles to have successfully made the shot. If I see the bird under those circumstances I will have not only missed, but also generally 'come out of the gun', meaning raised my head or 'peeked'. That is the one time, for me, where staying in the gun to the extent of consciously pressing my face into the gun is requisite. I shoot station 8 low gun skeet that way as well.

How much rib? Perhaps the better answer is to have someone knowledgeable help you evaluate where you are shooting the gun in question and adjust from there. Rocketman's and Salopian's remarks are both poignant.