To determine this utilize a ballistics calculator like http://www.jbmballistics.com or http://dop3.com enter the cartridge info and it will provide the drop so you can determine the amount of elevation required.

Keep in mind with scopes that the amount of correction is the full amount the scope moves and not from your zero. Based on the yardage you are wanting to shoot you will probably need to go to 25 or a 30 MOA base. Also shooting .308 at 1200 yds will be a big challenge and I hope you do not expect to do more than just punch paper.

http://egw-guns.com should have what you need. Also if you are really looking at shooting long range precision I would spend lots of time on Snipershide.com. Very good resource.