Loads and Frequency of shooting.
The best load is the one your gun likes most, which sounds silly but like most oversimplifications, is really quite wise. One of the fun things about bp is that it seems each gun has it's favorite load, and you get to play with it to find out what that load is.
O.S.O.K. is on the right track. But let me say at the outset, the key is being consistent, doing things the same way every time. Little things like how much you compress the load, or whether you tap the measure to get the powder settled, all matter in getting consistent results.
Start with a light load. 12 gr is a bit low; I'd start with 15. Use either a lubed felt wad or cornmeal to fill up the chamber so that the loading lever can seat the ball firmly on the powder; no, absolutely no, gap between the powder and the ball is allowed (safety issue). Shoot at least 3 and maybe as many as 5 shots, same point of aim each time regardless of where the ball goes. The point is to get a good group. Use a rest and a target no further away than 25 yards; 15 is better. Next, increase the load to 18 gr and repeat the process, then 20 gr, 22 gr, 25 gr and 30 gr.. I'd stop there, although you can load it higher. The likelihood is that you will notice a drop in accuracy (groups growing in size above 20 gr. Your optimum load will be the smallest group you get.
Lately I've taken to shooting a full cylinder at each load, and swabbing the bore and cylinder with a bp solvent between cylinders; this seems to give the most consistent results. Fouling can influence the results in extreme cases, so I just like to be sure it's out of the equation.
The best load for my .36 cal Remington New Navy, which is a Pietta, by the way, is 20 gr fffg real bp with a lubed felt wad and a 0.375 round ball. Sam Fadala's 'Black Powder Loading Handbook' doesn't list the .36 cal Remington, but he does show the most accurate load was 20 gr fffg and the optimum load (highest velocities and energies) was 30 gr fffg for the .44 cal gun, and he tested up to 40 gr, so it will definitely handle more.
I should mention that you need to check for a thin ring of lead to be shaved off the ball when seating it in the chamber. If you don't get that ring you may need to go to 0.380 round balls, which may mean you have to cast your own because they are hard to find. That would be most unusual, however it has happened.
Shooting once a week would not be a problem for a brass frame with the loads mentioned above. However, since you are going with the steel frame the issue is moot.