Offset Tracked Head Position Unity3D

Hi there,
does anybody know, where on my headset the tracked position is which I get in Unity3D? Is it right in the center or the front of the HMD?
What I want to do: I want to attach a Vive Tracker to the headset and I want to know the offset of my attached position to the tracked position of my HMD to get the trackers position as correct as possible.

I hope I could make myself clear and are happy to hear some answers.