I never casted yet bullets for rifle. Rifle cast bullets are finicky due to a lot of factors, speed, pressure, accuracy. A 308 goes over 2000-2500 fps and at that speed many variables are at play. At those speeds a gas check is required. I don't have the knowledge in this matter so I won't dare misguide you in any way. Anyway at over 2000 fps you will need minimum 22 BNH. I've never been there.
I cast only handgun bullets and never needed more than 12-14 BNH.
To get 13 BNH I alloy pure lead (that I bought from a company who sell lead alloys) with linotype(that I bought from a CGN-er). 95% pure lead with 5% lino. Simple and is right on the money without having to quench the bullets. If I quench I can get them even harder.
I love playing with lead and alloying things. It's so awesome.