A quantitative comparison between experiment and theory is presented, which shows that all ions of the Suzuki structure on (001) surfaces of Mg(2+) or Cd(2+) doped NaCl crystals can be identified despite the tip-surface distance, differences in impurity chemistry, and surface termination. The identification can be used to calibrate the potential of the tip's last atom, and it is proposed to use these surfaces for better characterization of deposited nano-objects.