Wow man thats actually a very big question. Much to do with familiarity and preference. There are people that can run a duplex out to a thousand, though I would not recommend that. Your leupold would not be a fist pick or even close, I believe that is a ballistic specific reticle so if your using a particular ammo it may or may not work with the reticle. My formula would be fairly straight forward depending on the caliber you are shooting. But let's imagine your using 308. The reason for my picks would be so you can do the ballistic calculations regardless of what type of ammo your running.
A front focal plane (FFP) optic in either 30 or 34mm tube. At least ten power, preferably fifteen. A quick rule of thumb would be one power for every hundred yards. A 20 moa mount so the optic and the ballistic curve align out from 800 to 1200 yards. And lastly a base ten so mil/mil reticle with matching turrets. Time to watch some videos on youtube regarding FFP optics and mill dot reticles.
watch this guy, he give some great primers on optics.
https://www.youtube.com/watch?v=TTca3wF35Og
for starter look at the:
SWFA super sniper HD
Vortex PST
Leuppold mark 4 or 6