For your pistol, 35 grains would be a good starting point.
Lots of controversy about what the Corps of Discovery carried. One thing for certain, it served them well. I have a replica 1803 Harpers Ferry rifle and enjoy it. I needed to restock it and a few other things but it is a good rifle.
The 1805 pistol is historical in it's own right. I believe a pair of them is used on one of the military decorations or badge. Not sure which one. Regardless, do not use the rifle load in the pistol.