Controlled low-strength material (CLSM) mixture design remains a trial-and-error process. A new approach using relative proportioning of the constituent materials instead of prescribed mass contents is proposed. Relative proportions allow for independent adjustments that enable unbiased estimation of their effects on CLSM properties. For the CLSM mixtures studied, a central composite experimental design was defined using three relative proportions: volumetric paste percentage (VPP), water-cementitious material ratio (W/CM), and portland cement-total cementitious materials ratio (OPC/CM). Second-order response models for slump flow, subsidence, and 28-day compressive strength were obtained for different sets of constituents, including virgin and recycled concrete fine aggregates and two fly ash sources. Slump flow and subsidence were most affected by the VPP and W/CM, respectively, whereas strength was explained by the combined effect of the W/CM and OPC/CM. The W/OPC ratio was not a reliable predictor of CLSM strength.