Thank you @Grant.
So by looking at this image on the link below:
Can you please guide me about how I can calculate the value R (the length of green line). I know by physics law that d = R*(alpha), and that alpha = arccos(v1.v2).
But not sure how to calculate the value of R, which is the distance of gaze origin to a 3D point in the scene.
I would like to see my changes in position of the eye in a plot similar to this one: