Top: normalized flux density distributions of 10 flux calibrators of Sgr A*. The dashed lines are Gaussian fits to the distributions. Bottom: standard deviation of flux densities of calibration stars versus flux densities of them. The red line is the polynomial fit to the measured σ values of the calibrators shown in the upper panel (red crosses). The two purple x symbols present the measured error values (obtained by Witzel et al. 2012) at the position of the comparison apertures for the background emission close to the position of Sgr A* (see Fig. 2).