Placement of Tefillin on the Head - How Precise Should One Be

A young man suffering from obsessive tendencies is undergoing treatment, thank God, all the difficulties he had have decreased or disappeared. However, regarding placing the tefillin exactly in the middle, he gets confused, it can take him a quarter of an hour or more until he finds the "exact" spot. According to him, he has not found a clear instruction that it does not have to be exactly in the middle and that a normal middle is sufficient, on the contrary, the poskim imply that it needs to be precisely in the middle. Can it be said how precise one needs to be - and how much precision is not necessary? And can sources be provided for this? Thank you very much!

Shalom and blessings 

Our teacher, the Gaon, Rabbi Avraham Fried, shlita, answered this: One should not be overly meticulous about the placement of the tefillin, as it is said in the Gemara (Eruvin 95b) 'There is room on the head to place two tefillin'. 

It should be added that without looking in a mirror, it is certainly impossible to be precise about this at all. And certainly, in all generations, everyone placed the tefillin according to the feeling that they are in the middle of the width of the head and did not look in a mirror, which was not common in ancient times at all. And it is not found in any of the rulings of the early and later sages that one is required to look in a mirror.

This is also sharply written in the responsa Divrei Chaim (Sanz). See sources. 

And here are the words of the responsa Divrei Chaim (Orach Chaim, Part 2, Siman 6)
If looking in a mirror to ensure the tefillin are in the middle is a matter of ignorance because even if they are not precisely aligned, it is kosher, as it is known that there is room on the head to place two tefillin [Eruvin 95b] even in width and there is no measure for the tefillin in width. This is the meaning of Mordechai [laws of tefillin 12a in the name of the Smag and 13a beginning "between your eyes"] explicitly [and see Tosafot Eruvin 95b beginning "place" and Beit Yosef, Siman 27, beginning "place of the head tefillin"] and the Gaon in his book was not strict in precision: 

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