For me Chair Pose is the same position as a squat in weight lifting terms. There are other versions of chair, but I think this version has the best and safest biomechanics. To start, I put feet hip-width distance apart. As I move from standing to chair, I start by hinging at the hip to incline the torso to about a 45 degree angle. The hips move back behind the ankles and the shins stay nearly vertical. The knees bend to wherever you can hold keeping them moving straight forward. NEVER let the knees roll inward! The deeper the knee bend the more difficult. Leg strength aside, for me chair pose is all about core control. Sitting the hips back behind the heels and bending the knees more is all about having the front of your body strong. I use my arms in front or overhead as a counterbalance. I cue to have people lengthen from the fingers to the hip creases. This helps to lift the lower belly and use the deep core.
If you want to increase the challenge, take the arms away by placing the hands just in front of the shoulders. Taking away the counterbalance sends the weight back and you need to squeeze even more in the core. To create this squeeze in the deep core start by lifting pelvic floor up, drawing the lower belly up and in, and then breathe broadly in and out of the ribs to hold the pose.
You can play with the rotation position of your hips: widen the feet and toe out a little. Be curious about where you now feel it.
Chair Pose is so important for everyday strength of getting up and down out of a chair. At work you should do about 30 repetitions of this sit to stand (stand to sit) motion every day. Add a couple of long holds just at the point before you let yourself sit down. Make sure you breathe and hold for as long as you can. This will help you build strength and endurance of the quadriceps (front of thighs), gluteus maximum (bottom), soleus (deep calf), and core (belly to spine action).
Happy Sit to Stand!