This paper reports a theoretical model of the optical gain of single-crystal scintillators of Nuclear Imaging. The model described the generation, propagation and escape of scintillation light as function of thickness and absorbed gamma ray energy. The latter was calculated via Monte Carlo methods at various crystal depths. The energies of 140 keV, 364 keV and 512 keV were investigated. The adopted thickness and energy values cover the range utilized in nuclear medicine imaging. For the semi-empirical approach, theoretical results were compared to experimental data for photon energies of 140 keV and 364 keV and the model's optical parameters were determined by the trial and error method. The results rendered the calculation of the optimum crystal thickness per investigated gamma ray energy. The presented results could be useful in designing nuclear medicine imaging systems.