I think you're looking for something that was not published or documented because it was not needed or even thought of at the time Jon.

The need for 'service pressure' standards arose with the development of smokeless powder.

With black, you charge by weight and what you get is what you get. Thus, the '3 dram' load.

Certainly different powders would develop slightly different pressure, but with a 200% overload being 'proof' whatever variation there is would be moot.