The ASTRI Mini-Array is an international project led by the Italian National Institute for Astrophysics (INAF) to build and operate an array consisting of nine Imaging Atmospheric Cherenkov Telescopes, currently under deployment at the Teide Observatory in Tenerife. They will study astronomical gamma-ray sources at energy above 1 TeV with unprecedented sensitivity among IACTs at E > 10 TeV. The ASTRI Mini-Array telescopes are equipped with a Cherenkov camera based on Silicon Photo-Multiplier detectors with an interferential filter as front window. They are an improvement of the ASTRI-Horn telescope prototype, operating at the M.G. Fracastoro INAF observing station in Sicily. As part of the ASTRI-Horn observational campaign, a fundamental calibration procedure was developed to determine flat-field correction coefficients. An experimental set-up was built with a stand-alone Cherenkov camera looking at the night sky pointing to the zenith without any optical system. By acquiring data with such device during particularly clear, moonless nights, pixel by pixel flat-field correction coefficients were determined to correct for differences in the filter transmission and Photo Detection Efficiency among the camera pixels. Flat-field correction can be applied to the variance data, that are related to the statistical analysis of the variability of the signals detected by the camera electronics, leading to a clearer vision of star fields with a consequent improvement of the pointing calibration procedure and of the monitoring of the telescope point spread function. The same procedure will be applied to the ASTRI Mini-Array to improve its pointing accuracy.