[en] New bio-metric equations used for assessing the thickness of the overlay plate to be added to the physical phantom were determined based on the Computed Tomography (CT) chest images of 33 adult males in order to improve the calibration of in vivo lung counting systems using the Livermore phantom. These equations are specific to systems composed of four germanium detectors with the measured subject in supine position. A comparison with the bio-metric equations used to date as reference in France was carried out and proved the usefulness of equations directly applicable to the Livermore phantom. (authors)