The two best powders for cast bullet shooting in 06, 300 mag etc is Trail boss, followed by 4759. For Trail Boss, fill to base of neck, weigh, start at 70% and work up till you find na accurate load. Most of my guns will shoot under an inch with Trail Boss and jacketed bullets, and with cast it all depends on the quality of the bullets but 1.5" is pretty common. You can also work down if you want to.
Reduced loads of 4895 also work very well (Hogdon site has youth loads listed that work with cast just fine, and loads of 30% less than published for any bullet weight work fine if there isn't a youth load for 200 grain bullets), if you can't get easy access to the other two.