The A2 upper receiver has 1 MOA, MOA = 1" at 100 yards, elevation adjustments and 1/2 MOA windage adjustments. The Military battle Sight Zero is done at 25 meters, 27.3 yards, by clicking up one click on the elevation knob and using the small aperture sighting the rifle in by adjusting the front sight post for elevation and the windage knob for windage.
When you click back down to the 8/3 mark the large aperture is used for targets from 0 to 200 meters. The small aperture will be zeroed in at 300 meters. By turning the elevation wheel up to the 4, 5, 6, 7 and 8 marks the rifle is zeroed for 400, 500, 600, 700 and 800 meters. These are approximations for those ranges and can vary with the ammunition and weight in grains of the ammunition as well being used.
Many people will sight in at 25 yards with the small aperture then refine the zero at 100 yards. Using the small aperture will give you a better sight picture for accuracy at any range. With the ammunition being used count the clicks it takes to shoot zero at longer known ranges. Note the information in a log book until you can memorize them for quick compensation at different distances. Then use that information for your range adjustments.