There is no perfect posture.
Mastery over posture is only realy achieved through mastery over samadhi.
Optimal posture
Relaxed erect sitting
Least muscular compensation
Least intra-discal pressure
Least pressure on internal organs
It's not about how it looks from outside, it's about how it looks from the inside, creating a clear basis of skeletal support.
Human spine is not strait.optimal posture has to respect the normal curves of the human spine.
Optimal posture begins from the ground up
Neutral hip position.
Sit on the Sitz bones(on the ground or with support). This is going to provide the maximum support coming back up the spine.
Use what is called the abdominal bracing procedure to set the spine, shoulders and head in to a neutral postion. And relax.
Neutral position of head and shoulders.
learn to relax more and more in to the posture.