Simply put the best way of sitting at a desk is to subscribe to the rule of 90’s. Sit with your hips, knees, ankles and elbows at 90 degrees and your computer monitor at eye level.
The truth is there is no perfect way to sit and sitting like this for hours in end will not save your posture. Be sure to move about and change your position subtly every 20-30 minutes. Basically you have 360 joints and 650 muscles in the body and they all need to move!