A method for forming an offset model is described. The offset model represents an estimated offset between a limbus center of a user eye and a pupil center of the user eye as a function of pupil size. The approach includes sampling a set of limbus center values, sampling a set of pupil center values, and sampling a set of radius values. The offset model is formed by comparing a difference between the set of limbus center values and the set of pupil center values at each of the radius values. A system and a computer-readable storage device configured to perform such a method are also disclosed.
