Hi @maryam, Assuming that you have stimuli points that the user is looking at, then you can calculate the distance between that and the gaze origin, which would get you the R value.
float dist = Vector3.Distance(gazeOrigin, stimuliPosition)
We also supply a ConvergenceDistance value in our API, but the accuracy of this signal might not be optimal for the type of study.
Hopefully this clear things up, let us know how you get on.