Relativistic runaway electron avalanches (RREA) play a key role in producing high-energy radiation in thunderstorm environments, yet their quantitative description remains largely empirical, with limited validation across atmospheric conditions. In this work, we develop a unified framework that consistently describes both avalanche development within the electric field and particle propagation beyond it, using CORSIKA simulations at four high-altitude stations spanning a wide range of atmospheric densities. We show that the classical relation for avalanche length requires revision: the empirical coefficient <em>K</em> is not universal but varies systematically with atmospheric density. Introducing density-dependent scaling yields a consistent description of avalanche growth across all sites. At the same time, we identify an effective energy-partition coefficient, calibrated at a characteristic propagation scale of 100 m, which remains stable across all stations and reflects the available propagation after exiting a strong acceleration field. The results demonstrate that RREA can be described as a two-stage physical system that links density-dependent avalanche growth with density-dependent particle transport via a universal energy-partition mechanism. This framework provides a compact and physically transparent basis for interpreting high-energy atmospheric phenomena across altitudes.