Looks like you're doing well to me. During art class my teacher always said during the shorter segments to concentrate on getting the movement of the figure. This is accomplished by capturing the movement of the spine, the long lines of the body in poses (the line from the arm down to the hip if a model is sitting with an arm in the are in a smooth arc), and general correct placement of limbs/head. If you get farther than capturing things like that, then great! But don't get bogged down in the detail.
Your proportions are awesome! Where the rib cage and hip bones are also important in figure drawing. Draw a little larger and get those two things right, I put dots where the elbow, knee joints are. Then get the tilt of the head, feet and hands. With these down you should be about to finish the drawing without a model. Add more detail to the chest to show where the center is back or front, this will tell you a lot about what its doing. Then if theres time i put a hint of the deltoid, the top of the femur on the hip, pubic bone and maybe two dashes for the eyes. If you draw large enough you can try to finish the drawing later with tracing paper. Good practice for your anatomy.
I really enjoy your 30 sec poses. I hardly ever get the proportions and the flow of movement but it looks to me like you get both rather well. I like to very my size on short poses because I think it gives a better overall understanding of the pose. Drawing larger also lets you fill in details and add character later. Awesome work.