I would agree with others in that you want it to shoot a bit high.
With that said, there is a problem in that you are sighting it flat down the rib like a rifle. Highly doubtful anyone is using that site plane while shooting. Typically when you have the rib flat and pattern, the gun will shoot low since a normal person mounting a shotgun and shooting will be seeing the rib on a rising to the bead which will raise the POI.
Can you confirm that you are looking down a "flat rib" while shooting? The ideal way to pattern is to not shoot the shutgun like a rifle, but to throw it up to your shoulder like you are shooting a bird, focus on the bead, put the bead on the target and pull the trigger with no thought to the rib.