Before you even think about powder you have to start with the projectile. Slug the barrel with a piece of soft lead and measure it with calipers. Compare this with the projectile you were hoping to use. What caliber is the bulldog supposed to be? .380 revolver is fairly common for these and the diameter for those is listed as .375 on this
Pistol Cartridge List this same list indicates the 38s&w is .359. The 38s&w would be much too small but you still have to slug the bore to be sure.
.380 revolver is also a heeled bullet design as I recall which makes reloading for it even more challenging. I use cut down 38 spl cases to get the proper OAL when I reload for .380 revolver.
If you buy an antique firearm and just stick whatever fits in the chamber and shoot it you are risking damage to the arm and to yourself. In the example above the projectile would be much smaller than the bore so you probably won't hurt yourself but it easily could be too big and therefore very dangerous.
ALWAYS slug the bore so that you know.
Until you get the size of the projectile correct, conversations about pressure are pointless. In this example of too small a projectile, most of the pressure is escaping down the barrel and going around the projectile as it doesn't fit. Accuracy and power would be greatly diminished.
The joy of a black powder load is that you don't need any formula of X grains of Y powder or a filler. You simply select the correct black powder (typically fff for a pistol) and fill the case so that it will be slightly compressed (no air) when you seat the projectile.