Example for the correction of poorly constrained coefficients for one star of the test set. Upper panel: scatter of the coefficients in a window of eight coefficients (black symbols) and errors of the coefficients (red symbols). The black and red lines give polynomial approximations to the scatter and errors, respectively. Lower panel: coefficients in the common XP basis, in the reference system diagonalising the covariance matrix. Black symbols: result of the calibration. Red symbols: corrected coefficients. Green symbols: true coefficients.