  Dust extinction and reddening are ubiquitous in astronomical observations and
are often a major source of systematic uncertainty. We present here a study of
the correlation between extinction in the Milky Way and the equivalent width of
the NaI D absorption doublet. Our sample includes more than 100 high resolution
spectra from the KECK telescopes and nearly a million low resolution spectra
from the Sloan Digital Sky Survey (SDSS). We measure the correlation to
unprecedented precision, constrain its shape, and derive an empirical relation
between these quantities with a dispersion of order 0.15 magnitude in E(B-V).
From the shape of the curve of growth we further show that a typical sight line
through the Galaxy, as seen within the SDSS footprint, crosses about three dust
clouds. We provide a brief guide on how to best estimate extinction to
extragalactic sources such as supernovae, using the NaI D absorption feature,
under a variety of circumstances.
