  Spherical Proportional Counters (SPCs) are a novel gaseous detector
technology employed by the NEWS-G low-mass dark matter search experiment for
their high sensitivity to single electrons from ionization. In this paper, we
report on the first characterization of the single electron response of SPCs
with unprecedented precision, using a UV-laser calibration system. The
experimental approach and analysis methodology are presented along with various
direct applications for the upcoming next phase of the experiment at SNOLAB.
These include the continuous monitoring of the detector response and electron
drift properties during dark matter search runs, as well as the experimental
measurement of the trigger threshold efficiency. We measure a mean ionization
energy of $\mathrm{W}=27.6\pm0.2~\mathrm{eV}$ in $\mathrm{Ne + CH_4}$ $(2\%)$
for 2.8 keV X-rays, and demonstrate the feasibility of performing similar
precision measurements at sub-keV energies for future gas mixtures to be used
for dark matter searches at SNOLAB.
