If I were in your situation, I would just let it grow out all at once rather than try to creep down. I tend to have the same issue with where my beard stops and I shave my head. I tent to attempt to get too close with the razor and trim a few beard hairs with every head shave. After the line gets too low for my liking, I just start over higher and let it grow in. Within a week it is hard to tell.

My Beard Journey:  http://jefffsbeardboard.yuku.com/topic/25137/Finally-I-have-a-Beard
Beard and Bald Thread:  http://jefffsbeardboard.yuku.com/topic/27831/Beard-bald-head-shaved-head-or-buzzcut
Patchy Beard Success Stories:  http://jefffsbeardboard.yuku.com/topic/33742/Patchy-Beard-success-stories-Before-and-After-photos#.ViujbH7nuM8
Taming the Wild Mustache:  http://jefffsbeardboard.yuku.com/topic/30822/Taming-the-Wild-Stache#.Vj_Mh7erTIU