Hey Chrystal, Good job working on the Loomis head. The thing I notice the most is that your lines do not wrap around the shape of the head. The skull is a 3D shape, so make sure you wrap the lines around it. I did a quick sketch to show you what I mean :)
I found myself making many of the errors that Stan talked about in his gesture videos before reading a suggestion by another user on the forums who mentioned skipping the timer and focusing on line economy. You could try re-watching Stan’s videos and then taking a pose and doing the bare minimum for lines. Circle for the head, single lines for the limbs, and then only a few lines for the important parts of the torso. Draw slowly and focus on making solid/flowing CSI lines. Try that for a few pages or sessions and then increase your amount of lines. Circle for the head, a line or two for the neck if needed, two strokes for each limb, and then more lines for the torso but still focusing on flow and staying away from only contour. Also, don’t be afraid of exaggerating the pose which is something I struggled with at first. Those are some things that have helped me recently.
You're not over-complicating: this is complicated stuff. Just like you can construct vanishing points for directions in the horizontal plane, you can do so in the vertical plane. For figure drawing, this becomes way too technical, and it is typically eye-balled...
John, I've just thought of the Andrew Loomis books. The "Figure Drawing" and "Creative Illustration" ones have very good sections on the use of perspective in illustration.
To be able to construct a perfect square in perspective, you need to understand a little more perspective, so you can construct the vanishing points for the diagonals of the square. using a 'stationary point'. It goes too far to explain it here, but google is your friend. Let me know if you need help, and I will see if I can find some pointers.
These are a good start. I suggest you draw 'through', like a wire frame, so don't just chop off one side, but also the other one, which is usually not visible. This helps to find the vertical middle line of the head, which I believe is your main problem for now. Also, consider drawing a little bigger, keeping your pencil sharp, to go for a cleaner construction.
I think that what we're seeing here is a sphere with three circles that cut it in two equal halves. The best way to approach this is to start with a cube, and then cut it in two, in three possible ways, and from there construct the circles (I will cook up an example today). Although this a good exercise, in practice most artists will simply eyeball this, and another good exercise is to tighten rubberbands around a sphere, like a tennis ball, and draw this as a model.