There are so many bullets for the .257 Roberts, that I am going to try to keep this to just the 100-gr bullets for starters.
I am no expert on these bullets, as I always used 117-gr or some 120-gr Federal Premium ammo for my rifle, until I bought some 100-gr Sierras to load for my Savage 99 in .250 Savage.
I started with IMR-4350, which was so good with 115, 117 and 120-gr bullets.
The charm of the .257 is that its mild factory loads are actually all you need for deer inside 200 yards. The Remington Core Lokt 117-gr RN is accurate, and kills with aplomb at 2,650 FPS MV.
The variety of brass, from thin to heavy +P, means you have to start low and use a chronograph. Also, IMR-4350 is a bit temperature sensitive. Where I live, the temperatures this year ranged from -4 to +109 F. I was not out shooting in all that, but a 50 degree difference is about 150 fps of pressure in the .257 with IMR-4350.
So start about 39.0 grains and check the velocity. If it is 2,800, then you might move up to 40.0. In cold weather, you might get up to 45.5 grains or more safely, but once the velocity gets around 3,000, you are topping out and probably groups opening up. 43.5 grains is probably going to be a good load in a strong rifle. But be careful and check velocity, and go slow past 2,900 fps in a 24-inch barrel rifle.
I want to try a Barnes TSX or TTSX in 100 grains, but all I have shot now are Sierra and Hornady, both close to 1/2 inch groups.