This paper addresses the problem of calibrating magnetically-coupled passive sensors for medical applications. We developed an analytical model of such sensor including the impact of some disturbances (distance and angle between antennas). This model is valid for sensors with different dimensions of antennas. Thanks to this model, the ideal indicator of the disturbances for calibration can be identified. It has been applied to the case study of Intra-Ocular Pressure (IOP) sensor. A new indicator (the slope of the baseline of the Bode phase plot) has been chosen.This new indicator is reliable to significant distance and angle variations between primary and secondary.