W.W. Greener, in the ninth edition (1910) of his book
The Gun And Its Development,
https://www.simonandschuster.com/boo.../9781616088422
...stated that a correct shot charge should weigh no more than 5 times as much as its black powder charge.
For a 12-bore gun firing 3 drams (82 grains) of black powder or the smokeless equivalent producing 1175 fps, the correct weight of shot is 410 grains or 15/16 oz.
Similarly, the minimum weight for a light game gun to mitigate against the effects of gun recoil headache is 100 times the weight of shot.
For a 15/16 oz. shot load a light game gun should weigh not less than 5.86 pounds.
Attachment 294785
The above define the parameters of the British Standard 2-1/2" 12-bore Game Load.