The theory behind kneeling chairs is that opening the angle of the thighs and torso results in better balance of muscular use and spinal strain. This works, however, only if the angle is not too minimal or too exaggerated. Look for a chair that has a slope of at least 25 degrees but no more than 30.
If you aren't sure what the exact angle of your main seat is, then look at the placement of your torso. If the angle of the seat is correct, you should lean slightly forward so that the torso is a little in front of the hips. Sitting further back in the seat is not encouraged because it may make you more inclined to slump; a slight lean forward makes slumping more difficult.
When you try to position the angle and height of your chair, always sit first. Don't be fooled into thinking that the chair is too small for you by sliding on the seat and relying on the knee seat to hold you. If your knees don't quite touch the kneeling seat while you are putting your weight on the main seat, lower the main seat until your knees are in a position to allow you to put some weight on the knees if needed, but don't let your knees completely press into the kneeling seat.
When you adjust the angle and height of your chair, do so in the location in which you actually will work. For instance, if you purchase the chair for use at work, adjust the chair to accommodate your work desk, not your home desk.
Kneeling Seat Adjustment
When you are sitting in the kneeling chair, check to see whether all of the kneeling seat is flush with your shin. If it isn't, adjust the kneeling seat angle (if possible) so that the whole top of the cushion is touching you. Otherwise, the force of gravity will pull down on a smaller area and you may have too much pressure on the shins.
If you buy a chair that felt comfortable in the showroom, ask an ergonomics specialist to evaluate how the chair fits you. If the chair feels too perfect, this may be a sign that the chair isn't doing enough to make you sit up straight, since sitting is much more active in a kneeling chair. At the same time, pain in the position that the specialist says should be ideal indicates a potential medical issue that needs to be addressed.