Following work done in the energy region above 100 keV, the high-precision calibration of a co-axial high-purity germanium detector has been continued in the energy region below 100 keV. Some of the previous measurements and Monte-Carlo simulations have been repeated with higher statistics and a new source measurement with  169Yb has been added. For the energy range from 40 keV to 100 keV, an absolute precision for the detection efficiency of ±0.2% has been reached, as previously obtained for energies above 100 keV. The low-energy behaviour of the germanium detector was further scrutinised by studying the germanium X-ray escape probability for the detection of low-energy photons. In addition, one experimental point, a γ ray at 2168 keV from the decay of  38K, has been included for the total-to-peak ratios agreeing well with simulations. The same γ ray was also added for the single- and double-escape probabilities. Finally, the long term stability of the efficiency of the germanium detector was investigated by regularly measuring the full-energy peak efficiency with a precisely calibrated  60Co source and found to be perfectly stable over a period of 10 years.