It's generally a style thing, and rear sight position is proportional to to barrel length.
The front sight ramp's leading/forward edge should be held back abut 1/4" from the muzzle end of the barrel. If the front sight's dovetailed directly in the barrel, the dovetail should be further back - almost an inch from the muzzle to the dovetail's front edge.
The rear sight should generally be located about 1/4 of the way towards the muzzle, from the receiver's leading edge, give/take. Tape it on, after the front's installed, and a position will suddenly "look right" to you, when view from the side. A sight, down the barrel, using the sights in the position you've selected, should tell you right from wrong.
Use Brownell's Sight Height Calculator to arrive at the correct sight blade heights - which may change as you move the rear forward and back, due to barrel contour.
http://www.brownells.com/aspx/NS/GunTech/sight.aspx