  Precise and accurate measurements of the beam energy delivered to the
experimental halls at the Thomas Jefferson Accelerator Facility (Jefferson Lab)
is required by many experiments for proper data analysis and physics event
reconstruction. During the 6 GeV era of Jefferson Lab, the energy delivered to
experimental Hall A was determined to 2E-4 dE/E with multiple measurements; but
after the machine was upgraded to 12 GeV, the accelerator's beam energy
calculations needed to be re-calibrated. In order to link the 6 GeV era
calibrations to the 12 GeV era, the Hall A ARC energy measurement system was
left unmodified. After the upgrade, this system was used to determine the
absolute beam energy being delivered into Hall A and find the new calibrations
for the main machine. To ensure the validity of these results, they have been
cross checked using elastic scattering data as well as spin precession data.
