Cox's formula - bore diameter times pi divided by .029 = twist rate
.775 x 3.14 / .029 = 83.91(1:84 makes a nice round number)

Rayl's formula - bore diameter times pi divided by .02618 = twist rate
.775 x 3.14 / .02618 = 92.95(1:92 is a nice even number)

I usually go with the Cox rate the faster twist is usually just fine for big bore rifles.
These formulas are for roundball rifles, for bullet rifling look up the Greenhill formula as a starting point.

As for loads I would probably start at 70 grains FFg and work up from there until I found the best accuracy.

I hope this helps.

Colin