Re: Bench shooting question
You can wrap your thumb or lay it along side whichever is comfortable to you, the main thing is to pay attention to how tight you're gripping, you don't want a tight grip, and you want to be able to press your finger straight back without shifting your hand. However free-recoiling a .308 sucks, so pull it into your shoulder with just enough force to be solid and deal with the recoil, this is one place a butt hook can be handy. Keep your support hand off the foreend, make elevation adjustments by moving the rifle fore an aft in the front rest and/or by adjusting your grip on the rear bag.
You can and ideally should use a benchrest type front rest and rear bag. If funds are tight and you have or are getting a bipod you can get good results with it and rear bag. When I started out I used sandbags, it might be low class and far from ideal but it works. A V type rear bag is ideal but a small bag made from a sock or whatever filled with sand and/or shot will work.