An expression editor tool allows a user to build an expression or calculation with a rule that specifies an action programmed to occur during execution of a target application. The expression editor tool allows a user to define an action point definition that specifies an action point located in the target application logic instructions. The expression editor tool determines from the action point definition a dataset that is in-context at the action point, provides an expression definition interface to obtain selected data fields from the dataset, and a selected logical operator to connect the selected data fields used to construct a new expression. The expression editor tool provides an outcome selection interface used to obtain a selected outcome for the new expression in order to create a rule including the new expression and the selected outcome, and store the rule in the database linked to the action point definition.