Generally speaking, Retumbo is the go-to powder for the 210vld. Although you should test H1000 just to be sure.
In terms of incremental powder charge changes, .3grain changes is too small. For a cartridge that has 89+ grains of capacity, a 1% change is .9grains. So for your initial testing, make 1 grain changes. Start around 88grains and go to 89, 90, 91. Once you find a load that seems like it's working, maybe go to a +/- .5grain change.
My 300RUM by MCR