For good handling, a shotgun should balance either on the hinge pin, or about three eights of an inch behind it ? but there?s more to it than that.
The main weight of the gun should be concentrated in the part between your hands ? in other words, between the middle of the fore-end chequering and the pistol grip.
If an over-under is muzzle-heavy, it can be improved by fixing a lead weight in the stock bolt hole, just under the butt plate.
However, such a cure will not make a gun balance quite as well as one that was built with good balance and handling in the first place.