If i remember right the way the nikons run is the 800 is good to about 600yds, the 1200 is good to about 800yds. what you really need to do is take the range finder to some place where you know the range of some game sized targets and then see what your rangefinder will pick up. coming from a guy who sales em and has tested a whole lot of them i would tell you the optimal range finder for the money is the Leica CRF 1200, it will get you consistantly out to 800yds on deer sized animals. The bushnell 1500 elite is also a really good unit for the money and while alot bigger than the little leica it will range a little farther.
the golden boy sittin on top of the mountain is the swaro. consistant ranging on game sized animals out to 1100-1200 yds and large targets in good conditions out to even 2000yds! its pricey but the only reason you would ever replace it is if they would make the aiming reticle a little better in it, or if you hit the dang lotto and could afford a Vector!!!!!
The main thing is to find a range finder that can give you CONSISTANT preformance at the range you are looking for. tons of rangefinders will range a target for you at 800yds one time or 1100yds another time, but in the long run when you need them they will fail you in that critical moment or worse yet give you slightly off range and cause a wounded and lost animal. you also need to find a rangefinder that will allow you to range smaller targets or allow you to range targets that are somewhat obscured. soemtimes the buck is standing between 2 trees and you may think you are ranging him but actually you have ranged the trees and he is 50yds behind them.
that is my long winded way of saying buy one of the high end range finders and dont look back. you will thank your self in the long run
take it easy
steve