16 pounds!!!!
It's supposed to pretty flexible according the manufacturer...but they're trying to sell the stuff so who knows
Online Hodgdon Manual
Shows a 100gr bullet getting pushed 2945fps with 39.5gr @44,800cup
Shows a 120gr bullet getting pushed 2695fps with 35.5gr @44,000cup
I'd start at 35gr and work up 0.5gr to 37gr...then go by 0.2gr.
But that's just me, the TRUELY smart thing to do is just buy a manual with .257 roberts/nosler/imr info.