The minimum acceptable crown-to-root ratio for a tooth to be considered periodontally stable is 1:1.5. This ratio ensures that there is enough root length embedded in alveolar bone to support occlusal forces and maintain tooth stability. A ratio less than 1:1.5 indicates insufficient periodontal attachment, increasing the risk of tooth mobility and failure under functional load. Maintaining this ratio is critical when planning restorative or prosthetic treatments to avoid overloading compromised teeth. Hence, a 1:1.5 ratio is the clinical threshold for favorable prognosis in periodontally involved teeth.

Reference: Carranza's Clinical Periodontology, 13th Edition.