Twist rate would be dependent on how heavy a projectile you want to send out there. And how far.
Heavy 425+ use a 15" twist
Medium 350 to 400 use a 16" twist
Light 290 to 350 use a 18" twist
Ultra light 210 to 290 use a 24" twist.
This would be for distance shiooting. For a close range all bets are off. But it would be in the ballpark. Try playing with the Greenhill formula from the late 1800's.
Note that all origional  Ballards in .38, .40, & .45 caliber used a 1 in 20 twist. I'm note sure on Sharps rifles.(talk about standardization)
 Don   rustyrelx

I use a 13 twist for distance (600 to 1000 yard) and heavy bullets.  For Silhouette use (up to 500 yds) I would use a 14 or 15 twist.  Standard twist used today is 16 and it limits your bullet length/weight enough to make the .40-65 non-competitive.  It would probably have been a lot more popular caliber and competitive with the 45-70 if it had been standardized with a 14 twist rather than the unfortunate 16 twist choice.

dave

I think the 40/65 as originally loaded was intended to be a higher velocity "Express" round in Lever Guns. Lighter bullets slower twist and lots of powder.   

What we are doing with the cartridge now is considerably different. If I was building one would follow the standard advice.

Chose the bullet according to ballistics required for the intended match. Barrel the gun to suit the bullet and use a case that just holds enough powder to launch it at the velocity needed.  Set up the rifle as heavy as the match rules allow.

Boats