Yo lots of great work so far! keep at it. I do note a bit of inconsistencies on the side cuts of the skull spheres, when drawing following Marc's take on the loomis method I find having almost perfect two-thirds proportion of the side cut circle is essential and when at angles, it can get a bit difficult as the angle of the ellipse starts changing. When done right you can tell because the overall head shape and jaw proportions seem to work.
Also, when designing the ramus and angle of the mandible note that the ramus tends to be a bit more vertical initially before rounding out - but ofcourse there's lots of anatomical variation.
cheers