  A general method for estimating the Yang-Yang ratio, ${\cal R}_{\mu}$, and
the coexistence-curve diameter of a model fluid via Monte Carlo simulations is
presented on the basis of data for a hard-core square-well (HCSW) fluid and the
restricted primitive model (RPM) electrolyte. The isothermal minima of
$Q_{L}\equiv< m^{2}>^{2}_{L}/< m^{4}>_{L}$ are evaluated at $T_{c}$ in an
$L\times L\times L$ box where $m = \rho - <\rho>_{L}$ is the density
fluctuation. The ``complete'' finite-size scaling theory for the
$Q_{\scriptsize min}^{\pm}(T_{c};L)$ incorporates pressure mixing in the
scaling fields, thereby allowing for a Yang-Yang anomaly.
