I just bought that very rifle. Loads aren't much of a problem with that thing...and it is accurate. Bought it because the same rifle in percussion is one of my favourites. But where I burn Pyrodex in the percussion Deerstalker, I use strictly real black powder (Goex) in the flinter. Reason being the real black (3f) is much easier to ignite with the flintlock system.
Using 3f you could shoot PRB or the lighter PowerBelts in loads from 50 to 80 gr. I think you could go as high as 100 gr to shoot PRB only.
The problem, if there is one with the Deerstalker, is getting the flint version to ignite as fast and as reliably as the percussion version.
It is capable of fast ignition, because it has done it. OTOH there is typically a delay between the pan charge ignition and the main charge. I use 4f black powder in the pan to give this thing every chance.
Besides 3f and 4f black powder, you might want to invest in a few different flints. The flint that came with mine did not work well and didn't throw many sparks. I now have a blonde French flint that does much better...but I don't know if it's good enough. This is my first flinter, so I'm still learning.
Don't forget to clean down into the patent breech & get rid of all oil there before going to the range. Don't forget to wipe the pan, frisson, and flint clean before every shot. Don't forget to check that the wedge is tight holding the barrel in the stock fore-end. My wedge was so loose I'm surprised I didn't lose it.
Bob