To enhance the budget estimations for a software project, especially for a major recurring customer, it is essential to review budget-related lessons learned from similar projects. This approach allows the project manager to utilize historical data and experiences to inform the current project's budget estimation. By analyzing past projects, the project manager can identify trends, anticipate potential issues, and adjust the budget accordingly to avoid previous pitfalls. This method aligns with the best practices of project cost management as outlined in the Project Management Body of Knowledge (PMBOKGuide), which emphasizes the importance of historical information and lessons learned for accurate budget planning.
References: (Project Management Institute's PMBOKGuide and Professional in Business Analysis Reference Materials)