It's always more prudent to go back to the manufactuer or supplier. I have two sources on Pietta and both are a bit different and they came with the same Rem.??!!
Here is my general rule of thumb. Understand that this is just a starting point and you will have to put in some range time.
On a long guns:
The Min. is the caliber and the max is two times the caliber.
Download the Max on 777. So, on a .50cal. The min. is 50grns by volume and the Max is 100grns. by volume.
On listed cap and ball revolvers:
The Max is the caliber divided by two.
Here is what my Pieta literature says, using Pyrodex-P
Be Safe !!!