Abstract: The first cutoff frequency of the Earth–ionosphere waveguide is a key parameter for characterizing the effective reflection height of the lower ionospheric D region, and its retrieval is highly sensitive to methodological choices. Using nighttime VLF electric field power spectra measured by the DEMETER satellite during 2006–2009, this study systematically compares critical procedures involved in cutoff frequency extraction, including two identification methods (the Toledo local minimum method and the V-shaped criterion), two statistical sequences (detect-then-average versus average-then-detect), and a continuous-point merging strategy. Seven representative latitude grids spanning 55°N to 55°S over the central Pacific Ocean are selected to evaluate detection performance and its impact on reflection height estimation. The results show that the Toledo method achieves higher detection coverage but with a relatively low accuracy (26.4%), whereas the V-shaped criterion yields fewer detections but a significantly higher accuracy (66.8%). The average-then-detect strategy fails completely at high latitudes and leads to a systematic underestimation of the cutoff frequency by approximately 15–40 Hz. The continuous-point merging strategy has a relatively minor influence, with differences of about 12 Hz. The global distribution of VLF reflection heights derived using the V-shaped criterion combined with the detect-then-average and merging strategy indicates that reflection heights over oceans are generally lower than those over land, persistent low-reflection-height bands exist over the equatorial Atlantic and Indian Oceans, and pronounced seasonal variations with amplitudes of 10–15 km occur at middle and high latitudes. Overall, the results suggest that the V-shaped criterion combined with the detect-then-average strategy is more suitable for climatological studies of cutoff frequency and lower ionospheric D-region reflection height.