Looking really nice! Loving these studies, I like the way you are constructing those noses
Arch looks good. It's going to vary from person to person as different people will have different chin thickness and mouth width, not to mention different ear to cheek to chin rhythms. I also kind of defines the limit of the entire "front facing" plane of the head box from what I can tell, while at the top lateral angle of the brow you can start to define the limit of the side of the head on the upper part (it tapers towards the inner circle you draw that measures 2 thirds of the bigger circle, and more or less defines the thickness of the upper lateral wall of the orbit)
I'm sorry if I'm being so intense with all this info, I'm going through some info overload studying these things now XD
cheers!