In order to calibrate the photoelectric photometry we need to determine the transformation coefficients of the colour equations linking the instrumental response of each band to the international standard bands of the photometric system adopted. This is accomplished by plotting some proper quantities and by computing (by means of least squares method) the coefficients of the colour equations. For more details read 'Astronomical Photometry' by Henden & Kaitchuck, or 'Photoelectric photometry of variable stars' by Hall & Genet.
At OAC we have done this tens of times; below you can see an example of the graphs obtained for each of the UBVR bands.