Hi @maryam, If you have some skill in programming then you can access this metric using the Tobii XR SDK where we refer to the user-eye tracking distance as the gaze origin, which originates from the eye. The 3D position of the eye relative the eye tracker can then be accessed by
TobiiXR.GetEyeTrackingData(TobiiXR_TrackingSpace.Local).GazeRay.Origin
We describe this in more detail @ https://vr.tobii.com/sdk/develop/unity/documentation/api-reference/
Hopefully, this should point you in the right direction, please let us know how you get on. Best Wishes.