As a general rule, if you hold dead on, your optimum sight in distance will depend on the size of the critical target area. With deer sized game, this is typically given as about 7 inches. If, for example, you are 2 inches high at 100 yards, dead on at 200 and 5 inches low at 300 hundred, then you are zeroed to hit that 7 inch critical zone out to 300 yards.
A 300 yard envelope is typically more than adequate for 90% of all hunting situations. If you are shooting past 300 yards, it's probably time to start thinking about target turrets and knowing your come up, IMO.
There are many software applications, including free ones on the net, that can help you determine your ideal zero.
Just for reference, the range withing which you can strike the vital zone without sight adjustment is called 'point blank range'. In the case above, aiming dead on will result in a strike withing the 7 inch vital zone all the way to 300 yards - hence a 300 yard PBR. The PBR will depend on the cartridge and the size of the vital zone.