Question:
How can I find out where makom shemocho shel tinok rofeis is with regard to the upper limit of the tefillin shel rosh?
Answer:
The Tur writes that makom shemocho shel tinok rofeis is the whole top of the head, (obviously it still has to be in the center of the head, between the person’s eyes), however it is preferred that it be from where the top of the head where it starts to slope down towards the face. All of this is better than putting the tefillin too low that it is below the hairline.
Best Wishes
Sources:
Tur O:CH 27, Biur Halacha 27-9 D:H Ad Sof.