By the way, modern black powder substitutes like Pyrodex work just fine in shotshells. Just be aware that when we say "70 grains", we don't mean it by weight, but by volume. In other words, "70 grains of Pyrodex" is not 70 grains weighed out on a scale but the amount a powder measure meant for black powder would throw if set to 70 grains.
Because the substitutes have a lower density than real black powder, measuring them by weight results in a serious overload.
A good rule of thumb as a starting point for black powder loads is to use the shot measure as a powder measure, equal volume of shot and powder. Many muzzle loading shotgunners use a Lee adjustable plastic shot dipper as both a powder and shot measure. You ask them what they are shooting and they'll tell you "1 1/8 ounce shot and 1 1/8 ounce powder" They really couldn't tell you their powder charge in grains.
Of course, here again, "1 1/8 ounce powder" does not mean literally by weight, that would be around 490 grains, something you would only shoot once even if the gun didn't blow up.