Hello there, i'm by no means an expert but I also experienced the same thing as you; where learning from draw a box is easier than learning on my own i was also learning figure drawing at the same time as draw a box. this is what I did: draw a lot of gesture drawing as exercises, i swear it really helps in observing through your naked eye. In 30 minutes you can draw 20+ sketches. Then critique after every page of sketch. A good figure drawing tells motion, is exaggerated, the center of axis follows the spine, the spine and legs are somehow connected. There is balance, and the body should be asymmetrical. Then you can proceed to actual figure drawing, learnings in draw a box can be applied in figure drawing actually. Try to see the shapes of the human figure, it it's still hard for you the 250 box challenge can really help, and also drawing from real life especially still life objects- drawing things around my house greatly helps improve my observations through naked eye, like a lot. draw a box helped me understand how form sits in space and a little perspective. As for checking my own figure drawings? I watch video critiques of Proko and review the books/pictures i've read/saw on figure drawing.

As for your way of approaching 250 box challenge that's really useful, your technique of placing your pen parallel to the existing line is not completely breaking the purpose of mark making. maybe you could use it to mark and place dots, as long as you still ghost your lines. no problem with that. and it definitely didn't defeat the purpose of challenge and it's not cheating. My tip in drawing boxes is yes one of it is compare the angles of each line in a set of parallels. 2. observe your first 3 "Y" lines on where they are going and ghost the lines parallel to them, after, place a dot to mark where the line should end. 3. always rotate your paper and observe the box in different angles.

and before you learn shading, learn how shapes sit in 3D space first, as this could be easier.

so in summary for my learning draw a box-gesture-figure-still life. after each mistake, analyze critique and draw again. You should also draw what ever you want so that you won't get burned out easily.

Remember: shapes, proportion, perspective, gesture--->colors,values, shading-->composition,anatomy,technique