Finding procedure of N for star HD122956, based on the residual minimization of T_eff, log(g), and [Fe/H] (Eq. (5)). Circles represent the parameter averages of N best-reference stars. The filled black circle indicates the values closest to the references. The blue solid and blue dashed lines are the average from the PASTEL catalogue and the standard deviation (Table 4). For this star, Fulbright (2000) published [Mg/Fe], and we compare this also with the averages as a function of N. All four parameters for the best N = 3 are compatible with the literature results.