[Zack The Ripper]

Personal preference and depends on the play environment.

Further forward and you have snappy target acquisition as having the sight forwards on the weapon allows your eyes to pick up the sight quicker from a low ready when sighting in on a target. This system works best for close to medium range.

Far back usually is reserved for medium to long range sighting as the reticle will be more sensitive to directional deviation. What I mean is moving the muzzle left, right, up, or down you will have more movement of the reticle to your eye with the sight further back than with the sight further forwards. This means little movements in the direction of your muzzle will be more apparent while aiming allowing you to compensate a little more accurately. This however is not used often with red dots as most people shooting medium to long ranges will use powered optics of some kind.

The general placement is a compromise between the two, with many mounting the red dot above or just slightly ahead of the mag well on the top rail.

Again, personal preference above all.