This work presents a threshold-based photometric framework for detecting instability onset in recurrent nova systems. The model introduces a proposed critical structural parameter (Φ_crit ≈ 11.1848), interpreted as a threshold indicator governing the transition from stable accumulation to rapid energy release. The framework uses observable magnitude gradients and variability metrics to identify pre-eruptive conditions in T Coronae Borealis. Current observational data suggests that the system remains in a stable regime, with a predicted near-term eruption window constrained through real-time photometric monitoring. The parameter Φ_crit is treated as a phenomenological threshold requiring further calibration against extended datasets. This study provides a testable, observation-driven approach for tracking instability evolution in variable stars.