If your rifle has the typical 577-450 chamber and throat, it will be pretty oversize, with a tapered section. After considerabe testing, I've found that a .467- .469 heavyweight (i.e. whch fits the throat) shoots best in these rifles with black, but you can get by with .462. BTW, The Lee dies are pretty good. As to measuring the slug, best is to rotate the bullet in the slack jaws of a caliper, and maybe add .001-.002.
And recoil? 85grs in an 8-pound rifle? Perfectly manageable! :-)