In this work, simulations of the clearance monitor HWM-1800 (Ludlum GmbH, Hamburg) with the Monte Carlo (MC) code FLUKA were performed and optimized. Nowadays, clearance monitors are being widely implemented for decommissioning activities in nuclear reactors and particle accelerator facilities. For every situation, a different calibration is typically needed, depending on the material composition of the samples to be released, their volume and the expected radionuclides. For this reason, MC simulations present a practical tool to obtain information on the capabilities of clearance monitors for different configurations. Early models of clearance monitors worked as counters, providing an energy-integrated value of the measured net count rates. However, modern devices have a multi-channel analyzer, which allows constructing pulse-height spectra. The results of the MC simulations were optimized for both of the described situations: comparing the integrated net count rates and performing a spectral analysis. The performance of both methodologies is evaluated in terms of their discrepancy with the experimentally measured values, and the possible improvements for the optimization with spectral analysis are further discussed.