In a previous study, the counting efficiency calibration was calculated for 36 computational mesh phantoms called MaMP and FeMP—Male and Female Mesh Phantoms, which span variations in height, weight and gender. They were created to be used in a sitting configuration WBC set-up using an HPGe detector. Now this work is extended to investigate the influence of body sizes and the position of the detector on the counting efficiency (CE) for a different counting geometry. This was done by creating a new set of MaMP and FeMP mesh phantoms lying on a reclining chair and by studying the variation of the counting efficiency using a NaI(Tl) detector for the WBC set-up of SCK-CEN. The deviation of the CE due to the detector’s position with respect to the phantom was investigated. Correlations were also studied for CE as a function of trunk volume, waist–hips ratio (WHR) and waist–chest ratio (WCR) for different mesh phantoms.