This calculator says 3.1" - and from my experience, it's just about dead-on all the time.

http://www.lmariana.com/traj.htm

* these numbers allow for no fudge-factor. That means, no splitting-the-difference holding the pin a little hot on the dot for the longer ranges.

Anyone who says their arrows are only dropping 1-2 inches between 20 and 30 is either shooting the fastest bow ever made, or they're cheating with the pin placement.