Precise Simulation of Near-Critical Fluid Coexistence

We present a novel method to derive liquid-gas coexisting densities, [Formula presented], from grand canonical simulations (without knowledge of [Formula presented] or criticality class). The minima of [Formula presented] in an [Formula presented] box with [Formula presented] 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 [Formula presented] to [Formula presented] of [Formula presented] down to 1 part in [Formula presented] of [Formula presented] (and confirm well Ising character). Pressure mixing in the scaling fields is unequivocally revealed and indicates Yang-Yang ratios [Formula presented] and [Formula presented] for the two models, respectively.