To answer your questions:
1. I think doing the circles with the cross is good. To make the feedback loop faster, you can grab some dividers and check yourself if you circles are really circular.
2. regarding the proportions of the head: you can also create your own feedback loop by taking a reference image of a head, construct it 1:1 by measuring out the proportions and landmarks and regularly double check them.
I am very big on creating your own feedback loops wherever possible, that way you can identify mistakes yourself without having to wait for the community to tell you. Sometimes coming up with a feedback loop is kinda tricky, but with the stuff you are currently learning, i think its quite doable.
If you have any questions about that, feel free to ask.