  A diverse array of science goals require accurate flux calibration of
observations with the Atacama Large Millimeter/Submillimeter array (ALMA),
however, this goal remains challenging due to the stochastic time-variability
of the ``grid'' quasars ALMA uses for calibration. In this work, we use 343.5
GHz (Band 7) ALMA Atacama Compact Array observations of four bright and stable
young stellar objects over 7 epochs to independently assess the accuracy of the
ALMA flux calibration and to refine the relative calibration across epochs. The
use of these four extra calibrators allow us to achieve an unprecedented
relative ALMA calibration accuracy of $\sim 3\%$. On the other hand, when the
observatory calibrator catalog is not up-to-date, the Band 7 data calibrated by
the ALMA pipeline may have a flux calibration poorer than the nominal 10\%,
which can be exacerbated by weather-related phase decorrelation when
self-calibration of the science target is either not possible or not attempted.
We also uncover a relative flux calibration uncertainty between spectral
windows of 0.8\%, implying that measuring spectral indices within a single ALMA
band is likely highly uncertain. We thus recommend various methods for science
goals requiring high flux accuracy and robust calibration, in particular, the
observation of additional calibrators combined with a relative calibration
strategy, and observation of solar system objects for high absolute accuracy.

Instrumentation and Methods for Astrophysics;Earth and Planetary Astrophysics;Solar and Stellar Astrophysics