This document presents Phase IIa of the RGG/T-Field project, establishing the weak-field calibration and foundational mapping between the Temporal Field (T-Field) formalism and observable dynamical quantities. The analysis develops the mathematical bridge connecting the theoretical structure of the T-Field to rotation-curve phenomenology, with a specific focus on deriving, validating, and constraining the universal acceleration scale parameter a0a_0a0. Phase IIa includes: • The derivation of the weak-field action and its correspondence to the RGG rotational geometry.• A systematic calibration method for extracting a0a_0a0 from observational inputs.• Consistency checks, limiting-case analysis, and stability criteria for the T-Field under weak curvature.• A coherent framework that prepares for the full empirical tests of Phase IIb (population-level analysis across small/medium/large galaxies). This document is part of a multi-phase theoretical and empirical investigation of the T-Field as an emergent structure governing gravitational and rotational dynamics. Phase IIa provides the mathematical and methodological basis required to interpret real astronomical data and to compare the RGG predictions with observed systems.