Vertically 0.5-1 cun.
When there is not sufficient Clear Qi (mainly the Lungs and Heart Qi) ascending to the head, the patient starts to loose memory and become sleepy during the day. On the other hand, when excessive amount of Qi goes to the head while not enough of it descending from the head, the patients suffers from insomnia.
This point is also used to treat other relatively serious mental emotional issues such as depression, psychoses or hallucinations. It is because it stimulates the Qi ascending and descending to and from the head, so as to open the Mind's orifices and calm the Corporeal Soul (Po).