Welcome to the world of semi-obsolete cartridges. The 222RM & 223 are really close in capacity with the Magnum being a BCH larger. I would think 223 loads would be a good start working carefully upward. If you compare the Nosler data between the 2 for Benchmark you'll see that, for some reason, they show the same powder charge with lower velocity in the larger case. The 4895 & H335 loads do not show equal charges or lower velocity for the Mag, I'm unsure what to think about the anemic RL10 load especially since Alliant shows 1 1/2 gr. more in 223 with 55gr bullets. It looks like 26 grains of CFE223 is a very safe starting charge for the Mag. As always... work upward carefully watching for pressure signs, maybe 1/2 gr at a time... or get some IMR4895.
