Based on the unique ZHK spacetime identity axiom R≡CT and the intrinsic 45° three-dimensional right-handed helical spacetime topology, this paper strictly deduces the mass-charge coupling constant k via gravitational and electrostatic field equations combined with the time-evolution coupling relation of fields.The constant k=√(4πε₀/G) is an inherent topological invariant of spacetime without any artificial fitting parameters, which geometrically binds the gravitational constant G and vacuum permittivity ε₀, realizing the underlying unification of macroscopic gravitation and microscopic electromagnetism.Two sets of values of k are given: the CODATA observational approximation k_obs≈1.2911492220 and the pure geometric locked true value k_locked=1.29128. The systematic dimensional defect error of classical two-dimensional physical model is quantitatively calculated, which explains the origin of numerous empirical fitting parameters in Standard Model.This work standardizes two dimension systems of k: primitive spacetime dimension L·T⁻¹ and SI apparent composite dimension C·s·kg⁻¹, and permanently abandons the flawed incomplete dimension notation kg/C.The tiny deviation between observed value and geometric true value comes from local spacetime distortion and artificial unit projection, rather than defects of the ZHK theoretical system.