For .223 I try for a ES of .3grns; and for .308 .5grns ES and .338 lapua 1.5grns ES. Not sure on the 300 RUM but I would imagine it would be close to the .338 Lapua, I would aim for 1 grn or so. I know nosler makes .300 rum that is what I would get.
If its a few hundred new cases, I would weigh them all and sort them that way I shoot groups of brass that have the same weight... sort of removes the problem