The Underground Muon Detector of the Pierre Auger Observatory features a calorimetric detection mode that estimates the number of muons based on signal charge measurements. This contribution provides an overview of the calibration procedure, revisiting the previously published strategy and identifying a bias introduced by the triggers used to estimate the mean charge deposited by a vertical muon. We demonstrate that calibrating underground detectors requires careful consideration of the interactions of penetrating particles through matter. In devices based on plastic scintillators, energy deposition—and thus the recorded charge—is significantly affected by knock-on electron production in the surrounding ground as muons traverse the medium. To mitigate this effect, we propose a new calibration strategy that ensures an unbiased muon estimator. This approach is applied to data collected from 2019 to 2024.