<p>Efficiency calibration is a critical step that enables the conversion of the detector count rate into the radionuclide activity in the examined parts for in vivo measurement. However, to date, there is no well-accepted framework for efficiency calibration. Thus, researchers often employ various types of anthropomorphic phantoms with considerably different patterns of radionuclide distribution coupled with drastically different numbers of detectors to derive the calibration factor, making cross-comparison among studies challenging. Moreover, some studies employ virtual calibration, whereas others prefer experimental calibration, although their equivalency has not been fully explored. In this paper, which focuses on in vivo measurements of ²¹⁰Pb in the skull, a promising technique for individualized reconstruction of radon exposure is presented, and a detailed framework for efficiency calibration covering the key issues mentioned above is provided. Specifically, physical phantoms of ²¹⁰Pb in the skull are developed on the basis of the anatomical characteristics of a Chinese adult reference male, along with the corresponding computational phantoms constructed from computed tomography scans. In comparison, the average deviations between the simulation and experimental results are within 4% for efficiency calibration at the top, left, and right sides of the head across varying detection distances. Furthermore, on the basis of investigations of ²¹⁰Pb distribution regions reported in the literature, the calculation method for ²¹⁰Pb activity is improved to account for different source distributions and joint measurements with multiple detectors. The results are useful for determining the appropriate conversion procedure under different measurement conditions.</p>