This was a very intense exercise, but it has helped me with my eye/hand coordination, and how to use negative shapes as a tool to assist in creating accurate drawings.
It took me 1 3/4 hours to get this far in the sketch. It tested my stamina because I am not used to drawing in this way, but I automatically see the difference using the methods you are teaching. I will make sure to do this exercise more often. 🙂