Function point analysis (FPA) is the best methodology to use for estimating the complexity of developing a large business application. FPA is a technique that measures the functionality of a software system based on the user requirements and the business processes that the system supports. FPA assigns a numerical value to each function or feature of the system, based on its type, complexity, and relative size. The total number of function points represents the size and complexity of the system, which can be used to estimate the development effort, cost, and time.

FPA has several advantages over other estimation methods, such as:

It is independent of the technology, programming language, or development methodology used for the system. Therefore, it can be applied consistently across different platforms and environments.

It is based on the user perspective and the business value of the system, rather than the technical details or implementation aspects. Therefore, it can be performed early in the project life cycle, before the design or coding phases.

It is objective and standardized, as it follows a set of rules and guidelines defined by the International Function Point Users Group (IFPUG). Therefore, it can reduce ambiguity and improve accuracy and reliability of the estimates.

It is adaptable and scalable, as it can handle changes in the user requirements or the system scope. Therefore, it can support agile and iterative development approaches.