This article is from the Ergonomics and Typing Injury FAQ, by Dan Wallach email@example.com with numerous contributions by others.
A. It is most important that seat height should allow the upper body
postures described in #7, #8, and #9. This upper body posture is most
responsible for reducing risk of injury. Once this is accomplished, the
feet should be flat on the floor.
If the resulting seat height prevents the feet from resting flat on he
floor, a foot rest is necessary. This should allow the lower legs to be
vertical and thighs horizontal.