A common situation faced by governments across countries is the mismatch between formally enacted reforms and their practical implementation. Traditionally, this gap is assessed by measuring the degree of institutional completeness, which reflects the extent to which reforms have been formally adopted. This study proposes an alternative approach by focusing not on adoption per se, but on the divergence between formal rules and day-to-day administrative practices. Drawing on the concept of decoupling—where institutional rules are symbolically adopted but not substantively embedded in operational decision-making—the research introduces a Symbolic Compliance Index (SCI). This index aims to systematically capture the mismatch between institutional provisions and administrative behavior. To explore this phenomenon, the study examines the reform trajectory of the Greek State Budget, with particular emphasis on the implementation of performance budgeting. The Greek case provides a compelling context due to the country's obligation to align with external institutional pressures from the IMF, EU, and OECD subsequent to the financial crisis. These conditions make Greece a paradigmatic case for analyzing the tension between formal compliance and the substantive integration of institutional reforms. The study’s objective is to illuminate the dynamics between institutional planning and administrative practice, offering a conceptual and methodological contribution to the study of public sector reform and institutional decoupling.