Over the years I've seen some pretty inconsistent lots of reloader 22 with some of them being pretty warm. Some 300 Win like my tikka have quite a bit of free bore and other ones don't. And different cases in the 300 Win Mag depending upon manufacture seem to have a pretty good variance of case capacity.
If I were in your shoes I would be starting right around 75 grains and getting it chronoed and go from there depending upon the velocity.