Get closer to this buck. Start working your way to longer ranges in the off season. Now is not the time to "learn" to shoot 600 yards. The guys that shoot long have been doing it a long time. I'm at 500 currently for my limit, and there's been a lot of time invested to get to that range. If you know where this deer is coming from and going to, set up closer. You don't gotta do it from a stand. They are just as dead when you shoot them from the ground. Good luck!
-Jake