Method for determining the FSPs, as applied in this case to a star with (log T_eff, M_bol)_star = (3.7,6.0). Left panel: first, for each star with a given [Fe/H], the masses and Ages of evolutionary tracks (continuous lines) are determined for a group of helium abundances (maintaining [Fe/H] constant) at the desired point. Upper-right panel: zoom-in of the left panel. Next, the helium abundances with reliable matches are selected. In this case, two Y values are disregarded i) Y = 0.230, because the star is fainter than the ZAMS for this helium abundance (dotted line); and ii) Y = 0.370, since a mass lower than 0.5 M_⊙ (the lower mass limit of the set of tracks) is required to pass through the point (log T_eff, M_bol)_star. Lower-right panel: since Y displays an almost linear dependence with log 1 at the given (log T_eff, M_bol)_star point, Y can be determined using the spectroscopic 1 value. The continuous and dotted vertical lines represent the spectroscopic log 1 value of the test star and its error, respectively. Dots represent the theoretical log 1 value at the point (log T_eff, M_bol)_star for each Y value, and their errors are obtained assuming errors in T_eff and M_bol of σ_Teff =  ± 50 K and σ_Mbol =  ± 0.05 mag, respectively.