  Mercury mission MESSENGER's %(MErcury Surface, Space ENvironment,
GEochemistry, and Ranging Solar Assembly for X-rays (SAX) observed almost 700
solar flares between May 28, 2007 and August 19, 2013, as cataloged by
\citet{dennisSOLARFLAREELEMENT2015}. The SAX instrument, part of the X-ray
Spectrometer (XRS), operated at 1 -- 10 keV, partially overlapping the energy
range of the GOES X-ray spectrometers. SAX provides viewing angles different
from the Earth-Sun line and can therefore be used as a GOES proxy for partially
or fully occulted flares as seen from Earth. For flares with GOES classes above
C2 seen on-disk for both instruments, we found an empirical relationship
between the soft X-ray (SXR) fluxes measured by both SAX and GOES. Due to the
different energy response of the two SXR instruments, individual events can
deviate on average by about a factor of two from the empirical relationship,
implying that predictions of the GOES class of occulted flares from SAX data
are therefore accurate to within the same factor.
  The distinctive GOES energy response in combination with the multithermal
nature of flares makes it difficult for any instrument, even other soft X-ray
spectrometers, to provide a GOES proxy more accurate than a factor of two.
