Well for me when I first got into ML I bought a .54 Renegade from a buddy who set me up with PRB over 95 grains of Pyrodex.
I went the opposite direction because while I'm not a small guy, honestly, that load just plain hurt after 8 or 10 shots!
I also had an inline, briefly & a whole other story, & immediately ramped up to the full magnum load of 150 gr(iirc?). WOW! Decided if I wanted to be kicked by a horse I could save on the powder and just walk down to the neighbor's pasture.
So now, I shoot 70-80 gr 777 under a PRB, enjoy it & don't flinch!

As an aside,until the ML bug bit 12 years ago I was never really a "gun guy" always preferring archery so I may be a little slow on the uptake here but just yesterday I finally realized something:

I took my old single shot, no name 20 ga shotgun out(love this gun I've had since 14 yrs old) to my range. Finally I understand about sabots!

I mentioned this in another thread, but I shoot 80 grains of Triple Seven with a 295 gr. Powerbelt. That combination is plenty of powder to do a lot of damage to a deer, plus I've read that Powerbelts don't like a lot of powder. It's the combination that seems to have the best accuracy for me. If I'm just plinking at the range, I use patched roundballs with 40 gr. powder to practice trigger control, sight picture and breathing. At 50 yards that combination hits maybe 3" max below a full load Powerbelt. And it's a lot cheaper and a lot easier on the shoulder.

But in the end, it's what combination of rifle, powder and projectile works best for you. There is no single correct answer.

I used to shoot 70 gr Swiss 3F in my 50 flinter. Made life easy, 1 can of powder, 1 box of balls, 1 bag of patches, 100 shots. Flints, around 5 over the coarse of that much shooting.

I currently use 90 grains of 2F under a 295 gr T-EZ, which I think is on the light side.

I've had 4 MLs. For each one I started at 70 grains and worked up in increments of 10 gr until the groups started to open up. This has always been 100 or 110 grains for me. So, in the guns I currently have, I shoot 100 grains.

IMO, for hunting you should shoot a stout load. You never feel the recoil anyway. I want the best (flattest) trajectory I can get, with accuracy, in the field. It is easy to under estimate the distance to the deer or whatever you are hunting. Every bit helps.

Target shooting is a different deal altogether.

I haven't tried BH209 yet and I don't foresee me using it either. Only because I shoot sidelocks. I only have one inline and that shoots a Pyrodex load very good. From what I've read, BH209 works best with heavy charges. My loads (except one rifle which is a powder hog) are either 80 or 90 gr of FFg, FFFg, Pyrodex RS or P depending on the rifle being shot. I have a T/C Hawken flinter that used 100 gr of FFFg. That's a stout load.

In general I use between 80 - 95 grains of T7. I use only 70 grains T7 for my dad because he has a bad shoulder and doesn't like recoil. No problems here so far. Now I have been loading 110 grains T7 in one of my rifles because it seems to be the most accurate powder charge so far.