HMD Tracking Origin - Where is it physically

Hello,
I'm encountering a bit of an issue in my development where the Z position of the headset increases and decreases based on the pitch of the HMD. This tells me that the tracking origin for the HMD is not located at the pivot point of my head, but elsewhere. Does anyone have a specific set of physical coordinates relative to the HMD for the origin point of the tracking system?