The wider the stance, the more stability, the less mobility.
Stating that the width of the stance is linked to the width of the shoulders is not good enough. The width of a persons shoulders is not linked to their height.
There is a direct link between the basic Frontal, Sidling and advancing stance where only one foot moves when changing from one stance to the other.
In the advancing stance we want to lower our centre of gravity to increase our stability.
However the knee of the rear leg should not pass over the toe.
The front knee should be slightly bent.
Therefore to find your individual ideal stance, start with the advancing stance and work backwards.
It is the same for the angle of the hips, knees and feet. They are rotated to a 45 degree angle in the advancing stance.
As only one foot moves when changing from the Frontal Stance to the Advancing Stance, both hips should be internally rotated to a 45 degree angle in the Frontal Stance.
There should be a posterior pelvic tilt. However some people push their hips forwards. This is called a forward shift and it not bio-mechanically efficient and will lead to injury.