Much has been written by many eminent designers and physiotherapists about sitting correctly.
In my experience it comes down to what works for you. Everyone is an individual with their own shapes and needs. If you understand the mechanics of sitting then you have a good chance of choosing the right chair.
Think of these stones; care is required to keep things working properly.
A convex seat with a firm pad allows you choice in all of these aspects of sitting. The image below shows how it all works for the Attentive chair. Positioned here in a neutral position with weight supported almost equally between feet, legs and sitting bones.
Knees below hips is a priority for a work chair. Elbows at about the same height, or a little above hands gives a comfortable position for using the key board. Flexibility of the pelvis allows a variation in the curvature of the spine. It also gives a sense of working the pelvic floor while sitting. Sharing weight between feet, thighs, sitting bones and back varies depending on whether you are leaning forward or backward. Something to be able to do easily for yourself.
If the back has a convex surface, it also provides support when leaning back. Oh that lovely feel of stretching your spine away from the habitual slump.
Don’t forget – colour is important too.