What we are considering here in gun-fit terms is 'drop'.
Side by sides tend to shoot lower than o/u guns and the correct amount of 'drop' for a side-by-side will generally show the iris of the shooter's eye resting on the rib when you look down the barrel from the muzzles (of a proven empty gun).
The more rib the shooter sees, the higher the gun will shoot.
A trap set-up o/u will see the eye higer over the rib than a Skeet set-uo o/u, in which the pupil will be resting on the rib when viewed from the muzzle.
You can play with this to adjust a gun to shoot higher relative to the point of aim.
A common flaw is where too much 'drop' is provided and when the shooter's cheek is firmly in contact with the face of the comb, as it should be for consistent mounting, the eye is looking into the top-lever. this leads to head raising, which leads to inconsistent, poor shooting.
