RE: 300 yard Sight In for a 300 Win Mag
I agree with nvmike when he is questioning your guide. No guide should let his hunter shoot at an animal at 500 yds. - even 400 for that matter. The 300 WM is very capable but at that range a lot comes into play. I don't know where you live but the altitude difference may also play a role in trajectory. And what about wind. At that range, a 20 mph crosswind would throw your bullet off considerably (a 180 gr., 30 cal bullet would probably drift about 15" - figuring on about 3" per every 100 yds.). I'm going on a mulie/antelope hunt in October. My outfitter told me to sight in 1 1/2" high at 100 yds. This would be just fine with a standard crosshair scope. I have the Burris Ballistic-plex reticle and I've shot out to 300 yds using it and I am pleased with the results.
I would strongly suggest to your guide that you are not comfortable taking a shot over 300 yds.
As for your 300WM. I would sight-in 3.5" at 100 yds. that would pretty much allow you to hold dead center on an elk's chest out to about 325 yds.
Remember, the closer the better. I hope you shoot a nice bull at 100 yds. Good Luck.