Reply To: Tefillin Shel Rosh poll

The entire bayis she rosh must be above the (original hairline). For the back, vertically it must be in the center of the head (like the bayis in the front- bein einecha, as pointed out by the moderator) and horizontally the upper part of the kesher should rest on the bone at the base of the skull and the lower part of the kesher should rest on the indentation/nape of the neck. Minimally, it needs to be entirely above the neckline.