Developing analytic equations of state for fluid mixtures based on perturbation theories requires simplifying approximations, in which mixture properties are determined from effective pure component properties. While these one-fluid approximations reduce model complexity, they also introduce inaccuracies. In this work, a simple algebraic correction factor for perturbation theories applying a one-fluid approximation is proposed. The correction factor is defined as the ratio of the rigorous first-order perturbation term in Helmholtz energy to the respective first-order perturbation term in the one-fluid approximation. An approximate closed-form expression for the correction factor is derived in terms of the fundamental measures of the particles using symbolic regression. The new approach is thoroughly evaluated by applying it to a range of equations of state, including the uv-theory and the PCP-SAFT equation of state, to predict the thermodynamic properties of square-well, Lennard-Jones, and real-substance mixtures. New molecular simulation data for strongly size-asymmetric binary, ternary, and quinary Lennard-Jones mixtures are generated to test the proposed approach. Compared to the predictions obtained from a one-fluid approach, the correction factor significantly improves the accuracy of predicted phase equilibria and thermodynamic properties for model fluids. Its impact on real-fluid predictions with the PCP-SAFT equation of state is rather small, because PCP-SAFT segment size parameters of most substances are rather similar, whereas the correction factor primarily accounts for segment size asymmetry.