The Full-disk vector MagnetoGraph (FMG) instrument will carry out polarization observations at one wavelength position of the Fe I 5324.179 angstrom spectral line. This paper describes how to choose this single wavelength position, the relevant data-processing and the magnetic field calibrations based on the measured polarization signals at one single wavelength position. It is found that solar radial Doppler velocity, which can cause the spectral line to shift, is a disadvantageous factor for the linear calibration at one wavelength position. Observations at two symmetric wavelength positions may significantly reduce the wavelength shift effect (similar to 75%), but simulations show that such polarization signals located at the solar limbs (e.g., beyond the longitude range of +/- 30 degrees) are not free from the effect completely. In future work, we plan to apply machine learning techniques to calibrate vector magnetic fields, or employ full Stokes parameter profile inversion techniques to obtain accurate vector magnetic fields, in order to complement the linear calibration at the single wavelength position.