Event-B provides a promising feature of refinement to gradually construct a comprehensive specification of a complex system including various aspects. It has unique difficulties to design complexity mitigation, while obeying Event-B consistency rules, among the potentially large possibilities of refinement plans. However, despite of the difficulties, existing studies on specific examples or high-level and intuitive guidelines are missing clear rationales, as well as principles, guidelines or methods supported by the rationales. In response to this problem, this paper presents a method for refinement planning from an informal/semi-formal specification. By defining primitive rationales, the method can eliminate undesirable plans such as the ones failing to mitigate the complexity. In a case study on a popular example from a book, we derived an enough small number of valid plans only by using the general and essential rationales while explaining the one presented in the book.