I make muzzleloader for my living and I was a barrel maker at one time too. I still earn my living making muzzleloaders but no longer make my own barrels. But I do understand these things pretty well.

There are "formulas" that give you mathematical calculation for twists, but from experience and just looking at that bullet I would make twist 1 turn in 38" and I would cut the barrel grooves .004" deep. You will find that if you cut grooves 1.5 times wider then lands and .004 deep the accuracy will be very good. Some rounding of the corners at tops and bottoms of the lands and grooves both is helpful in such barrels. Sharp corners do OK and can be very accurate, but if you are going to pick up lead in such bores it's common for it to gather on the corners first, so having a bit of rounding eases that propensity.

If making a muzzleloader it's best to have the your top land of the mold cut .001" over groove diameter so it engraves on loading and the rest of the shank of the bullet cut 001 under land diameter. It gives enough drag to keep the bullet against the powder when hunting.

If making a breach loader, make the bullet drop at groove diameter.

With a 1-38" twist you will be able to run powder charges giving velocities from about 900 FPS up to about 1800 FPS with good accuracy. Faster speeds will need a stronger alloy however.