Gunny is correct. There's much to be figured out first.
Which cartidge is the right one for that rifle? They each have their favorite. Have you sighted the rifle? Sight at 25 yds. then at 100 yds. Once you've verified the sight is accurate, and you have the correct cartridge, give it a go. I'm not familiar with the drop for a .308, but, I'm sure it can be found in the ballistics information for whatever cartridge you use.
Then, account for wind, ambient temperature, humidity... you get the idea.
Having said all that, I have placed a 4 shot group at 3" from 400 yds with my .270
That rifle likes a 150 gr Winchester Power Point cartridge best. It has zero drop at 400 yds and is dead on for 2 magazines. After that, they tend to spray a little. Let it cool down and it's good for 5 or 6 more. But, I know this, so, it's not a problem.