Good question!
The box is intended to be a simplified version of the hips, and the cricles represent where the bone legs attach to the hips.
Knowing this, it depends on how big your box is, but usually they should go near the lower corners, right in the middle.
Here's what I mean:
Of course it's just inteded to be a guide, and as such, to simplify some people simply place them higher at the side of the box, or some on the lower side.
Techincally, the right position is as above, but this will mainly come into play the more you get into anatomy.
At the moment, in term 1, just getting a general idea of the dynamics of body gesture is good enough, since gesture plays such an important role at posing.
Accuracy on their positon will come into play once you start dressing up the gesture with muscles and bones.
Edit: I should have added that you seem to have them in the right place in the last excercics you posted! So great job at that!
