Can you be more specific as to the reason you're looking for a new seatpost? Type of bike, recent riding history with regard to fit, what you're trying to correct/adjust, saddle specifics, etc...

With this type of information, you'll get some great feedback. Typically, the amount of setback varies with the geometry of the frame's seattube angle, the frame size in proportion to the rider's ideal dimensions, the configuration of the saddle rails, and the desired fit characteristics the rider seeks. These can combine to give you a range (between saddle rail adjustment and setback) that will work for a particular bike.

So, what bike is it and how does it fit now?