(1) Background: Maximal fat oxidation (MFO), its associated exercise intensity (Fatmax) and the cross-over point (COP) are known indirect calorimetry-based diagnostics for whole-body metabolic health and exercise. However, large inter- and intra-individual variability in determining their corresponding intensity makes their use inconsistent, whether the intensity is based on power output or oxygen uptake. Blood lactate concentration (BLC) has often reflected a range in MFO and COP, which may offer another non-indirect calorimetry dimension based on the near equilibrium between lactate and pyruvate at the molecular level, which biochemically determines an interchange between lactate and relative rate of carbohydrate (relCHO) and relative rate of fat utilization (relFAO). This paper proposes a new testing approach describing relCHO as a function of BLC, with an individualized half-maximal activation constant of relCHO (kel), to explain and predict the variability in MFO, Fatmax and COP. (2) Methods: Following ethical approval, twenty-one healthy males participated in the incremental cardiorespiratory maximal test, and capillary BLC was measured. Indirect calorimetry relCHO and relFAO were calculated, and a constant kel that reflected 50% of CHO saturation level was estimated as a sigmoid function of BLC (mmol·L−1): relCHO = 100/(1 + kel/BLC2). (3) Results: 86% of relCHO variability was explained by BLC levels. The individualized kel estimations, which were 1.82 ± 0.95 (min/max 0.54/4.4) (mmol·L−1)2 independently explained 55% MFO and 44% of COP variabilities. Multiple regression analysis resulted in kel as the highest independent predictor of Fatmax (adjusted r-square = 22.3%, p < 0.05), whilst classic intensity-based predictors (peak power, maximal oxygen uptake, fixed BLC at 4 mmol·L−1) were not significant predictors. (4) Conclusions: The BLC-relCHO model, with its predictor kel explains the inter- and intra-individual variability in MFO, its exercise intensity Fatmax and power outs at COP through dynamic changes in BLC, fat and carbohydrates regardless of the intensity at which exercise takes place. kel capability as a predictor of MFO, Fatmax and COP independently of their associated intensities provides a new diagnostic tool in physiological exercise testing for health and exercise performance.