The problems of determining the abundances of elements in the atmospheres of F-K stars by the synthetic spectrum method are discussed. The algorithm used is based on the minimization of differences of the residual fluxes r_v = F_v/F_v^cont in observed and synthetic spectra. It is realized in the program ABEL8 and is suitable for a wide range of astrophysical tasks. Results of the use of the procedure for modeling λλ 669—672 mn in the spectra of the Sun (G2 V) and the fast rotating (V⋅sini = 90 km/s) red dwarf (K2 V) from the system V471 Tau are presented.