In recent years, arrays of radio antennas operating in the MHz regime have shown great potential as detectors in astroparticle physics. In particular, they fulfill an important role in the indirect detection of ultra-high energy cosmic rays. For a proper determination of the energy scale of the primary particles, accurate absolute calibration of radio detectors is crucial. Galactic calibration – i.e., using the Galaxy-dominated radio sky as a reference source – will potentially be the standard method for this task. However, uncertainties in the strength of the Galactic radio emission lead to uncertainties in the absolute calibration of the radio detectors and, thus, in the energy scale of the cosmic-ray measurements. To quantify these uncertainties, we present a study comparing seven sky models in the radio-frequency range of 30 to 408 MHz. By conversion to the locally visible sky, we estimate the uncertainties for the cases of the radio antenna arrays of GRAND, IceCube, LOFAR, OVRO-LWA, the Pierre Auger Observatory, RNO-G and SKA-low. Finally, we discuss the applicability of the Galactic calibration, for example, regarding the influence of the quiet Sun.