There may be a foreshortening issue with the photo on the monitor. We are looking down slightly on Kirk in that photo. That makes his face appear a little squatter than the sculpt. When compared with dead on profiles, for example, the sculpt is exactly the same length as Kirk.
One of the techniques I use (as well as good old fashioned measuring!) is to hold the sculpt up to a photo and overlap it then allow my focus to pass back and forth from sculpt to photo, sculpt to photo. This helps keep the sculpt in line with the proportions of the subject. However, perhaps there is a slight width difference across the cheeks or jowls? Of course, it doesn't help that Shatner tended to put weight on during the series. Most of my reference was captured from "The trouble with Tribbles" along with other photos garnered from the net.
Please explain about the forehead a little more. Where do you see it as concave?
That's the idea, here, an open exchange of ideas to make it a better sculpt. I expect you guys to keep me on my toes... 
