From what I can see that Sightron scope has 50MOA of adjustment (it's not clear on the Sightron website...)
So, you have 25 up and 25 down, assuming your more or less zeroed when at 25MOA "up".
My ballistics chart says I need about 31MOA to get to 1000, and 50MOA to get to 1300.
A 20MOA rail will get you 45MOA of dialing up. (25 in the scope plus 20 in the rail), and leave you 5MOA of dialing down 25MOA in the scope munis 20 in the rail) which should be ok to allow you to zero at 100yds. If you go to a 30MOA rail then you have -5MOA of down dialing ability - which doesn't exist in this dimension, so unless you know Sheldon Cooper, you're hosed.
I'd give up on anything past 1000 with a .308. You're subsonic at 1200, and then it all goes to hell.
If you get a boomer that's good to WAAAYY out there, you'll need a scope with more adjustment, likely in a 34mm tube.