SR4759 is a great all around powder for the MLII. My personal fun loads end up around 36gr-39gr of N110 with a 225gr bullet. N110 is a tad more temp stable than SR4759 especially when we are talking about reduced loads. N110 produces quite a bit more peak pressure too unless you have the old metal can 4759 which is closer to N110 grain for grain. Ive never tried lower loads with 4759 in a 50cal but it worked pretty good in a 45cal conversion. It would easily beat BH209 speeds without blowing sabots.

In my custom built 50cal it takes about 3-4gr more of 4759 to match N110 in speed but 4759 peak pressure is still less with more common load data. (Savage Book Loads) Much less if shooting a 300gr bullet.

This load came highly recommended by the former owner of the Modern Muzzleloader forum. It was listed as one of his most accurate loads. You could probably switch out to a cheaper 225gr for recreation. Yours may need a tighter fitting sabot.
Barnes 225gr .451 XPB
34-36gr N-110
Harvester short black sabot
CCI 209 primer