I just wanted to expand a little more on what rraisley said. The two yardages your sights will be on are determined by two things. 1) The specific ballistics of the bullet you are shooting. Some have a flatter trajectory while some have more of an arched trajectory. 2) The height above barrel line your sights are set at. These two factors will affect the yardages, but 25 and 75 are a general rule of thumb.