  This is the last of three papers describing an `absolute' calibration of the
GONG magnetograph using and end-to-end simulation of its measurement process.
The simulation begins with a MURaM 3D MHD datacube and ends with a `synthetic
magnetogram' of the corresponding magnetic field values as they would be
observed by GONG. We determine a calibration by comparing the synthetic
magnetic field measurements with the MURaM magnetic field values that produced
them. The previous two papers have described the GONG measurement process (both
instrument and data processing), our simulation of it, and the theory of
magnetogram comparison and calibration. In this paper, we address some final
points on calibration, combine all of this work into a set of calibration
curves, and consider the results. We also review the results of the previous
two papers for locality of reference. Our calibration indicates that GONG
magnetograms underestimate weak flux by a factor of over 2 near disk center,
but that factor decreases to about 1 as the line-of-sight approaches the limb.
A preliminary investigation of the generalizability of these results suggests
other instruments will be affected in a similar way. We also find that some
differences in previous magnetograph comparisons are artifacts of instrumental
resolution which do not reflect an intrinsic calibration difference, and the
measurements are more similar than sometimes thought. These results are
directly applicable to question of solar wind prediction model accuracies,
particularly in the search for the cause of the common discrepancy between
predicted solar wind magnetic flux at 1 AU and values measured in situ by
current satellite missions.
