This paper discussed the importance and necessities of formula defined by end users according to their own requirement of a system. It is believed that almost all information management systems should supply such a module during it's developing cycle. After a brief introduction and Comparison of formula define to the common methods currently used by many management systems, it selects one typical of them analyze thoroughly and illustrate data model of the system. Finally, we take the agent Commission management system, for example to illustrate the calculation process, formulas resolution rules and procedures.