mehavey
Why would you want to [re]size for Rim headspace rather than for shoulder fit ?
A) The first shot of a piece of brass will be off the belt and not the shoulder. New brass come from the factory with the shoulder pushed way back.
To get around that you could mandrel up the neck, then partial neck size, and get a tiny shoulder that will stand up to the firing pin, but I don't know anyone doing that.
B) There is 0.010" variation possible in shoulder headspace from rifle to rifle per SAAMI tolerance. A piece of brass shot in one rifle, resized, and fired in another, could easily wind up spacing on the belt again.
C) You can't screw up and push back the belt too much, but you can have the die screwed too far into the press [too close to the shell holder when the ram is raised] and push the shoulder back too much. It may take as little as 0.005" shoulder push back to be on the belt again. With 7/8-14 thread on a die, the pitch is 0.071", so 0.005" adjustment is only 4 minutes if the die were a clock.... and you would be on the belt again. You can bet that mistake happens every day.