The chair was perfected by the Egyptians thousands of years ago and has not varied much even with modern ergonomics.

 First your feet have to comfortably rest on the floor, so if the chair is adjustable, adjust it to where your feet feel comfortable on the ground.

 Next measure from where the chair seat starts to ends, your feet should comfortably hang over the side of this. Take that measurement and divide it by 4. Now take the quotient and measure from the back of the seat of the chair to the quotient.This is the lean you should have on your chair.

Measure from the bottom of the chair to the where the seat lies, take that number multiply it by 2 and divide it by 3, this is the height that the seat back should be.  

The armrests should line up with tail bone and should run about 1/4 of the seat length back from the front. The armrest should be at a height of about your kidneys

The perfect chair is relative to your size, therefore the perfect chair or perfect chair setup for someone else will likely not be the perfect chair for you(unless your the same size and proportions)
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This techinically applies to all chairs, but because this is a computer site, I made it about a computer topic.

Source
Anatomy for Interior designers 3rd Edition written by Julius Panero