When sitting in a chair, we must mimic the hip position of seiza. In seiza, our thighs are angled at a slight decline because the folded feet elevates our bottom slightly above the legs.
So the important point is to sit on the front edge, with only your bottom touching the chair. (you can sit deeper than what’s shown in the picture).
If we sit in the middle of the chair, our legs will be resting at the same level as our hips. Ideally, our legs must be free from the chair in order to angle slightly downwards, thereby elevating our bottom slightly higher.
Don’t slouch!