In this work, we present a novel methodology to evaluate the maintainability of Structured Object-oriented Formal Language (SOFL) specifications, addressing an existing gap in maintainability assessment for formal specifications compared to source code. We then introduce a structured rating methodology based on a weighted Z-score approach to define and quantify essential maintainability factors, including analysability, complexity, modifiability, and testability. Additionally, we implement a supporting tool that automates this evaluation process, making the SOFL maintainability assessment method practical for real-world applications. Our results demonstrate that the proposed methodology and tool provide a comprehensive assessment, effectively identifying weaker modules within the specification and establishing a basis for pinpointing potential issues.