I'd ramp it up a bit just to be safe. Maybe 70gn FFFg or 80 of FFg as a minimum. I am looking into .50 cal loads as well. What amazed me was how little difference there was in downrange velocity between loads. Having said that, not all our shots are at the maximum range are they? I am looking to work out an accurate load around 75 - 85gn of FFFg so that I can use the same powder for prime.
Looking at the info in a Gun Digest blackpowder loading manual for a .50cal Green Mtn barrel, the muzzle velocity with 50gn of FFg is 1250fps. With 90gn FFg it is 1710fps. That is 460fps difference, but at 100yards the difference is only 190fps. At 100yds the difference between a 70gn & 90gn load is only 75fps but 200fps at the muzzle.
The law of diminishing returns kicks in pretty hard as you increase powder charge so any real benefit of heavier powder charges probably only exists in the first 50 - 60 yards.
I don't even bother looking at energy figures any more. I know that pure lead ball will flatten out perfectly every time and do the appropriate amount of tissue damage, so I am more interested in accuracy and penetration.
Hope that helps.