This paper presents a procedure for reconstructing the longitudinal magnetic field from observational data of the complete profile of spectral magnetic lines. The method is based on a fast search algorithm for curve fitting for complete profiles of magnetically sensitive lines 6301.5 Å and 6302.5 Å. The magnetic field is calculated as the distance between the centers of the I -profiles of different polarization components. The centers are defined as the midpoints of the Gaussians approximating the I + V , I – V profiles. This approach makes it possible to carry out calculations rather quickly. The presented method makes it possible to reduce the noise in maps of a large-scale magnetic field and improve the accuracy of measurements, primarily of weak fields. The method has been tested to reconstruct the large-scale magnetic field from observations with the STOP full-disk magnetograph telescope (Kislovodsk) in 2014–2022.