Actually, the closer your sight is to the bow, the closer your pins will be together.

As far as your " one pin" idea goes, for my rig, if I were to use a single pin at 25yards,

At 20 yards, I would be about 2.5" high and at 35 yards it would be about 6" low.

Check out the link below, it will allow you to plug some numbers for your bow and determine the arrow trajectory.
http://dialspace.dial.pipex.com/town/terrace/qq53/gbag.applet1.shtml#Java%20Trajectory

I have used this a number of times before, and it seems to be spot on with my setup.

Just as a matter of personal preference, I prefer at least 2 pins (20 & 30). This allows me another reference mark when gapping the pins for a 24, 27 or 35 yard shot.