We used the data of 701 stars covering the colour index interval 0.32 < B − V ≤ 1.16, with metallicities −1.76 ≤ [Fe/H]≤+0.40 dex. The data were taken from the PASTEL catalogue and estimated metallicity-dependent guillotine factors, which provide a more accurate metallicity calibration. We reduced the metallicities of 11 different authors to the metallicities of Valenti & Fischer (2005), and thus obtained a homogeneous set of data which increased the accuracy of the calibration, i.e. [Fe/H]= −14.316δ20.6 − 3.557δ0.6+0.105. Comparison of the metallicity residuals for two sets of data based on the metallicity-dependent guillotine factors with the ones obtained via metal-free guillotine factors shows that metallicities estimated by means of the new guillotine factors are more accurate than the other ones. This advantage can be used in the metallicity gradient investigation of the Galactic components, i.e. thin disc, thick disc, and halo.