The decision of which granulation of powder to use has always been a curious question.
As others have mentioned, there are certain rules of thumb regarding caliber and use. However, there are exceptions to those rules. David Little of the Gun Works (Springfield, OR) shoots 40-grains of 2F in his 40-caliber Ohio Flintlock rifle. As the story goes, David ran out of powder at a shooting match years ago. The person he was with (Gun Works Founder, Joe Williams, I believe) loaned him some 2F. His groups immediately shrank with many shots being in the same hole. He’s used it ever since.
For years I preferred 3F in everything as I was told it burned cleaner with less fouling. I was out test firing a new to me CVA 50-caliber Hawken rifle I won on Gun Broker. Both 3F and 2F had the same Point of Impact (POI), but the 2F had less recoil, especially with a Crescent butt-plate. I now use 2F in anything 50 and up, unless the barrel is short such as 24”. I use 3F as it burns faster and hopefully more complete in the shorter barrel. While along those lines, would a slower burning powder such as 2F be more advantageous in a longer barrel (30” or more), regardless of caliber? According to Ned Roberts’s book, “The Muzzleloading Caplock Rifle”, the target shooters of the mid to late 1800s used 1F Curtiss and Harvey powder imported from England in calibers 38 and up. Granted, they were shooting paper-patched bullets.
I’ve been playing with a new, in-the-box Traditions Crockett rifle, settling on 20 grains of 3F. My next trip to the range will find me measuring 30-grains of 2F for a comparison.
Rifles all have individual personalities and idiosyncrasies (quirks); they like what they like and no two are the same. I actually have two Crocketts. The first was “dead on” right out of the box. The second needs the sights adjusted. If you tear them down, there are matching numbers on the tang, trigger guard, and stock. The mortises for these guns were made to fit those parts. The only thing without a number is the lock. I’ve even found where those are not interchangeable, even in the same model of gun.
To finally answer the OP’s question, I would start with 3F as the barrel is something like 24” if I remember correctly.
Good Luck!
Walt