  Utilizing data from the $Solar$ $Magnetism$ and $Activity$ $Telescope$
(SMAT), analytical solutions of polarized radiative transfer equations, and
in-orbit test data from 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 {\sc i} $\lambda$5234.19 A,
working line used by FMG. From the experimental data of SMAT, it is found that
the most sensitivity position is located at the line center for linear
polarization (Stokes-Q/U), while it is about -0.07 A away from the line center
for circular polarization (Stokes-V). Moreover, both the theoretical analysis
and the in-orbit test data analysis of FMG prove again the above results.
Additionally, the theoretical analysis suggests the presence of distinct
spectral pockets (centered at 0.08-0.15 A) 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 A
emerges as the champion for routine FMG magnetic-field observations.
