Looks interesting! I don’t know much about the black-smithing process, do you mean that the bar gets twisted axially into a sort of fusili-pasta shaped thing before being wrapped around the larger diameter form?
Does it get rolled into a flat circle shape first and then elongated vertically or do both actions happen at the same time?
I think what you’re looking for may correspond to the integral of the torsion of the curve along its entire length… although of course there’ll be all sorts of real-world complications compared to an idealized curve with zero thickness 
Edit: Here’s an idea, you could try comparing the Perpendicular Frame (zero-twisting) component with the Curve Frame at the beginning and end of the curve, and see how much they diverge from each other.
I believe that should theoretically give you the total amount of twist, or at least provide an approximate starting point where you can tack on whatever necessary correction factors 
See the attached: spiral.ghx (160.2 KB)
