Lots if powders work well in the 308. RL15, Varget seem to be favorites but others have had good success with other powders.
OAL will depend on two things
How far will you be to the lands and do you want it to feed from the mag.
If you want mag feed, you can't load longer than the mag.
If you don't care about mag feed (or are using a single shot) then you have to set the bullets based on the distance to the start of the rifling. Opinions vary as to how far from the lands or if you should be jamming the bullet into the lands. Really depends on the rifle too.
Just starting out, I would say work with OAL provided in the manuals (which should work in the mag) and then experiment from there.
Oh, and 155's are good to a 1000 yards usually. The Amax has a bit of low BC compared to others so unless you drive it really fast (3000 fps ish) it can go sub-sonic before you get there (trust me, I know

) and then things get weird.
I am going to try Lapua Scenars and maybe Berger 155.5's this year at long distance to see if I get better results. I am going to stick with the cheaper Amax for precision (800 yds max) this year I think though.