hey man, im in a similar position too but we can do 10 things at one time so, we do on thing at a time and then move on to next one.

so i would say do figure drawing for as long as you can without adding contour lines, checkout proko's figure drawing on youtube, love life drawing on youtube and https://www.proko.com/course/figure-drawing-fundamentals/overview,

stan is a good master at what he does, his courses are pricey but worthful.

so, try it out if u want, some stuff is free, some paid.

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

For figure drawing a lot of people recommend Micheal Hamptons book. It is very good but doesn't explain things to the beginner. However I recently came across his lectures on youtube and they really helped. Might be worth a try.

https://www.youtube.com/channel/UCHNinW31ADYz1SRrYuSKPBA/videos

Remember, none of this is easy. Breaking through the resistance is difficult but with enough purposeful practice you will see progress. It tends to require focus with purposeful practice on one aspect for a bit then put it aside and focus on something else. Rinse and repeat.

As for the pen/ruler thing. Not quite the same as using a ruler if you are not using it draw but pushing yourself to judge by eye only should be the objective.

Hope this helps.