It is recommended to sit all. You can't just sit on the edge of the chair. The weight of the whole body is there. After a long time, it will lead to fatigue and deformation of the buttocks.

Only 2/3 of the chair.

Keep your back straight, occupying 2/3 of the chair, and spread your strength to your hips and thighs. If you want to lean back in a chair, please choose a chair back that can fully support your back strength.

A.try to combine your legs. The posture of opening legs will affect the shape of pelvis for a long time.

B. When sitting on tiptoe, it is very beneficial to tighten the hip line.

C. Yes, but try not to sit cross-legged for a long time, otherwise the blood circulation will be bad.

This is an excerpt. You can look at the resources ~ ~ I hope it will help you!