First id like to say well done, the head is not a easy task, and its good that you have refrained from added detail to the skin. What id advise is rather than study the reference images, study the Skull. The anatomy of the skull will help you understand the base structure and will help you understand what you cant always see in fleshed out reference images. Anatomy is going to not only help you understand the bones that make up the skull, but their relationship , to each other. Proportions are essential, and can be wrong even if all the bones of the skull are in place. Here are some generic things to look for which do vary but keep you initially within a tolerance.
Head height,Width and Depth:
From chin to peek point of Cranium divide it into 8ths, this can be done with Transpose tool in Zbrush. From the Glabella (Between the eyes) (in line with brow bone, side profile) to back of Cranuium is 7/8th. Width 3/4 of the height of the head, iv found in some cases 2/3. highest Peek point of the cranium is normaly above, behind the ear, or above a bone called the Mastoid Process. Widest point of the skull is is whats called the parietal eminence, its located behind the ear, above the ear, I place like to place just below the Frontal Eminence (Forehead) if a line was drawn from it. Widest point of Face is the Zygomatic arch which is the side of the cheek bone. The skull is wider than the face so when you look at it from the top view your see it taper from back to front of face.
Generally if we draw a line from the side profile of the head both the horizontal and vertical this sets us up to get the rough placement of the eyes via horizontal line. Behind the vertical line is the location of where the ears start from, and in front of that line the Jaw (Mandable) . Draw a line at the brow bone, and below the nose, the ear height sits somewhere between these lines, this does vary of course. The back/bottom of skull Occipital bone is in line with bottom of nose. Hair line to brow bone is same measurement from Brow bone to bottom of nose, as from bottom of nose to chin. This varies from person to person. Eyes can fit 5 side by side from front view, be aware of perspective it will throw things off, so turn this off in Zbrush when measuring. Human eye is about 24mm, Iris half of that.
Tear duct lines up with wing of nose, corner of mouth varies a huge amount but is in line either with the pupil or inner edge of Iris. The corner of mouth is also where the K9 teeth are which denotes a plane change for the teeth front to side plane
These are all general and are not rules, they help a artist keep within the bounds of normality/averages. You can use these to get the difference between the “ideal” and reality or likness of a person.
Stick to low polygon forms to get the base measurements right, and main land marks before moving on. Sculpt a Skull, or draw a few, this will help you get the forms and plane changes in there even if you cant see them on the reference image of the person your sculpting.