There are four points of flexibility in present molecular formulations of the corresponding states idea. At these points the procedure followed or the numerical values can be tailored to achieve better agreement between predicted and experimental properties without violation of physical principles. These flexibilities are the exponent values used in the pair-potential function, the averaging procedure used to obtain an effective mixture pair-potential function, the combination rule for unlike-pair parameters, and the selection of a reference substance. The first two flexibilities are used in this paper to show that the conformal-parameter equations can correlate excess free energy and activity coefficient data of highly nonideal liquid mixtures. A pair-type-probability function is introduced in the averaging procedure used to obtain a mixture pair-potential function. Any macroscopic or molecular property may be used in this weighting function to improve mixture-property prediction.