Calibration of a High Purity Germanium (HPGe) detector with a certified reference material is valid for the specific geometry and matrix that was used during calibration. If the certified reference material contains nuclides emitting gamma photons in cascade (coincidence summing), this will have an effect on the efficiency calibration unless this is corrected for. The effect of coincidence summing is increasing with a shorter distance to the detector. When measuring samples that differs from the calibration geometry or matrices, the attenuation of the photons and consequently the activity determination will be affected. Therefore, these differences have to be corrected for. In this report the approach for establishment of a coincidence free near geometry calibration is shown. Further, the use of four different computer codes for calculation of correction factors for systematic effects within gamma spectrometry was evaluated.