1. Your English is fine, you're at least as good as me and it's my ONLY language!
2. Don't agonize over trivial things like cast bullet diameters until you can consistently get decent accuracy with jacketed bullets.
Your excellent Marlin 336 is a medium range game in deep woods rifle and you'll be shooting game bullets; neither are target grade so be reasonable about accuracy. I expect 2" groups or slightly better @ 100 yards from most Marlin 336 rifles and a reliable point-blank killing trajectory to 150 yards or a little further.
Full charges of IMR 4064 or Varget are my most consistently accurate lever gun powders. If you can't get one of them perhaps you can find something with nearly the same burn rate.