I would suggest that if you are serious about 400+ yd shooting--get a range finder with a distance uphill/downhill correction.Optilogic makes this kind of rangefinder and others do too.

If you do not correct for uphill/downhill angles at diatance the bullet will always shoot high for distance.Its that critical.A 15 degree angle can make a rifle shoot 2 inches high at 450 yds.

If you know the ballistic compensation for the rifle/scope combo you are getting close--The only thing left to screw you up is the wind!! It can be blowing one way where you are and the other way where the animal stands. At a measely 5 mph it can move a high BC bullet 7 inches @450 yds.If there is ANY wind noticable in the scope around the animal-DON'T SHOOT!!

Optical aids don't compensate for technique and rifle quality-practice, a good rifle with a good trigger,and a proper distance compensating scope matter more than most will admit.You have to do it regularly to get confidence--

Still its great to shoot under proper conditions and hear the SMACK of the bullet hitting the animal!!It is very doable with practice and reasonable judgement.