  It is challenging to achieve high precision energy resolution for large
liquid scintillator detectors. Energy non-uniformity is one of the main
obstacles. To surmount it, a calibration-data driven method was developed
previously to reconstruct event energy in the JUNO experiment. In this paper,
we investigated the choice of calibration sources thoroughly, optimized the
calibration positions and corrected the residual detector azimuthal asymmetry.
All these efforts lead to a reduction of the energy non-uniformity near the
detector boundary, from about 0.64% to 0.38%. And within the fiducial volume of
the detector it is improved from 0.3% to 0.17%. As a result the energy
resolution could be further improved.
