It is governed to a large extent by the physics formula F=MxA where force equals mass times acceleration. When max shot charge was used to denote proof level powders were pretty much standardized in terms of burn rate resulting in similar peak pressure and overall pressure curve shape. Thus if the mass is specified i.e 1.125 oz, the accelaration is pretty much a given (at that time) say 1200fps then the force applied to that mass to accelerate it 1200fps at muzzle would tend to be constant. The force is the product of the area under neath the pressure curve, as powders then had fairly even burn rates peak pressures would fall within a safe band. It began to fall apart with the introduction of shells like Eley Maximun which had 1&3/16th oz shot but were safe in 2&1/2" guns marked 1&1/8th oz max. They acheived this by modifying burn rates to lower peak pressure but extend the elevated portion of the curve further down the bore.
