Utilizing data from the Solar Magnetism and Activity Telescope (SMAT), analytical solutions of polarized radiative transfer equations, and in-orbit test data for the Full-disk Magnetograph (FMG) onboard the Advanced Space based Solar Observatory (ASO-S), this study reveals the magnetic-sensitivity spectral positions for the Fe i $\lambda $ 5234.19 Å working line used by FMG. From the experimental data of SMAT, it is found that the most sensitive position is located at the line center for linear polarization (Stokes Q/U), while it is about −0.07 Å away from the line center for circular polarization (Stokes V). Moreover, both the theoretical analysis and the in-orbit test data analysis of FMG confirm the above results. Additionally, the theoretical analysis suggests the presence of distinct spectral pockets (centered at 0.08 – 0.15 Å) from the line, harboring intense magnetic sensitivity across all three Stokes parameters. Striking a balance between high sensitivity for both linear and circular polarization, while capturing additional valuable information, a spectral position of −0.08 Å emerges as the champion for routine FMG magnetic field observations.
