  We present a novel method to derive liquid-gas coexisting densities,
  $\rho^{\pm}(T)$, from grand canonical simulations (without knowledge of $\Tc$
or criticality class). The minima of $ Q_{L}\equiv< m^{2} >_{L}^{2}/<
m^{4}>_{L}$ in an $L$$\times$$L \times$$L$ box with $m = \rho - <\rho>_{L}$ are
used to generate recursively an unbiased universal finite-size scaling
function. Monte Carlo data for a hard-core square-well fluid and for the
restricted primitive model electrolyte yield $\rho^{\pm}$ to $\pm 1$-2% of
$\rhoc$ down to 1 part in $10^4$-$10^3$ of $\Tc$ (and confirm well Ising
character). Pressure mixing in the scaling fields is unequivocally revealed and
indicates Yang-Yang ratios $R_{\mu} = -0.04_{4}$ and $0.2_{6}$ for the two
models, respectively.
