  Percent-level measurements of the comoving baryon acoustic scale standard
ruler can be used to break degeneracies in parameter constraints from the CMB
alone. The sound horizon at the epoch of baryon drag is often used as a proxy
for the scale of the peak in the matter density correlation function, and can
conveniently be calculated quickly for different cosmological models. However,
the measurements are not directly constraining this scale, but rather a
measurement of the full correlation function, which depends on the detailed
evolution through decoupling. We assess the level of reliability of parameter
constraints based on a simple approximation of the acoustic scale compared to a
more direct determination from the full numerical two-point correlation
function. Using a five-parameter fitting technique similar to recent BAO data
analyses, we find that for standard {\Lambda}CDM models and extensions with
massive neutrinos and additional relativistic degrees of freedom, the
approximation is at better than 0.15% for most parameter combinations varying
over reasonable ranges.
