WHATS A GOOD POWDER AND CHARGE FOR A GOOD POWDER CHARGE FOR CAST BULLET

Really good question.  Thinking that bullet to bore fit is important to cast accuracy I would suggest a borerider design to flatten the learning curve just a bit.  Second up is to consider the short .304 case neck on the .308 case.  The case neck plus the jump to the freebore + whatever length the freebore is will put the bullet into the leade/lands.  You'd rather not have the gas check below the case neck.  That makes the borerider design a bit easier to start with.  

Sometimes looking at past history and match results can help.  Here's all the .308 Winchesters that were shot the 21, 22, 23 Nationals.  The sort is by 1) powder, 2) mold used.  Right now I'm working with IMR4227, 4198, RL10X.  Most often repeated is 4895, RL7.  5744 and I are taking a trial separation at this time.  Powder wise I don't think there's a fixed "go to" powder in my mind.  There's some flexibility there.  Bullet to bore fit seems to trump most everything as far as accuracy goes.  Almost universally the velocities are in the 1600 - 1750fps range.  So if you have a couple powders in mind, maybe whats already on your shelf, get the charge figured out for 1650, 1680 and 1710fps.  That narrows down the time to get a load dialed in. It looks like IMR4227 is pretty popular with borerider designs.

The Egan design's from Accurate like the 210E and the 230E are bore riders and you can have the mold cut to fit your bore.  Yep forgot something.  I'd recommend a pound cast first off to see how deep your chamber is (case mouth to freebore face), the freebore diameter and the length of the freebore before the leade starts.  

Last tip would be to ask if someone on the forum could send you a few samples of a bullet that you might be interested in so you can do a test fit to see how the bullet fits your bore.  PM messages work great if you don't want to go public with the request.  Good luck and if there's anything I can do to help let me know.  Bill C.