While game-based assessment offers new potential for understanding the processes students use to solve problems, it also presents new challenges in uncovering which player actions provide evidence that contributes to understanding about students' knowledge, skill, and attributes that we are interested in assessing. A development process that combines the agile methodology common in game design with the principles from Evidence-Centered Design can provide a means by which to develop game-based assessments with clear assessment arguments. This paper presents an example of such an approach using four iterations that result in an increasingly more refined evidence model linking game activities to a student model.