Guys, it's the same as shooting a rifle with amill dotscope. Sight in that rife and scope to hit center bulls eye at 100 yards and then take at shot at 2 yards from the muzzle and use the center of the crosshair. The bullet will impact low on the target the same distance your crosshair is from the center line of the bore.

Now do the same thing but use the bottom mill dotand you should impact very close to dead center at 2 yards because your rifle will be pointing higher in the air in relation to the target.