Unity ARKit estimate neck and shoulder position from head tracked location

ShoulderAnchorManager.cs

//Dynamically calculated head size values upon arkit face added event
//Estimated head height used to determine head's pivot
[System.NonSerialized]
public float headHeight = 0.0547f;
//Estimated head depth used to determine head's pivot
[System.NonSerialized]
public float headDepth = 0.0505f;
public float headWidth = 0.0447f;
//Shoulder width should equal ~ 2x head height
[System.NonSerialized]
public float shoulderWidth = 0.15f;

void FaceAdded (ARFaceAnchor anchorData)
{
    trackedEntity.SetActive (true);

    faceMesh = new Mesh();
    faceMesh.vertices = anchorData.faceGeometry.vertices;
    faceMesh.uv = anchorData.faceGeometry.textureCoordinates;
    faceMesh.triangles = anchorData.faceGeometry.triangleIndices;

    // Assign the mesh object and update it.
    faceMesh.RecalculateBounds();
    faceMesh.RecalculateNormals();

    //Set the head size using the mesh bounds as an approximation
    headDepth = faceMesh.bounds.extents.z;
    headHeight = faceMesh.bounds.extents.y;
    headWidth = faceMesh.bounds.extents.x;
    //Shoulder width is two head heights for an average adult
    shoulderWidth = faceMesh.bounds.extents.y * 2.0f;
}

void FaceUpdated(ARFaceAnchor anchorData)
{
    //Estimate the position of the pivot of the head to reduce swimming effect when applying shoulder offset value
    //1.Get position of center of face 
    //2.Translate down from center of face using headHeight
    //3.Translate away from camera using headDepth
    var faceUp = (UnityARMatrixOps.GetRotation(anchorData.transform) * Vector3.up).normalized;
    var faceForward = (UnityARMatrixOps.GetRotation(anchorData.transform) * Vector3.forward).normalized;
    var facePosition = UnityARMatrixOps.GetPosition(anchorData.transform);

    neckPivot.position = facePosition +
                            (faceUp * (headHeight * -0.5f)) +
                            (faceForward * headDepth);

    //Determine forward direction vector towards camera from face
    cameraLoc = cameraRef.transform.position;
    forwardDir = (neckPivot.position - cameraLoc).normalized;

    //Vector3 shoulder estimates
    shoulderLeftBaseEstimate = neckPivot.position + ((shoulderWidth * -1.0f) * Vector3.Cross(forwardDir, new Vector3(0.0f, 1.0f, 0.0f))) - new Vector3(0.0f, headHeight, 0.0f);
    shoulderRightBaseEstimate = neckPivot.position + (shoulderWidth * Vector3.Cross(forwardDir, new Vector3(0.0f, 1.0f, 0.0f))) - new Vector3(0.0f, headHeight, 0.0f);

}