Question: when sitting at a computer you should?
Answer: Push your hips as far back as they can go in the chair.
Adjust the seat height so your feet are flat on the floor and your knees equal to, or slightly lower than, your hips.
Adjust the back of the chair to a 100°-110° reclined angle. ... 
Adjust the armrests (if fitted) so that your shoulders are relaxed.