Construct maps and related item type frameworks, which respectively describe and identify common patterns in student reasoning, provide teachers with tools to support instruction built upon students' intuitive thinking. We present a methodological approach--embedded within common assessment development processes--for developing such construct maps and item type frameworks which entails (1) articulating a construct map, (2) developing diagnostic item types, (3) comparing item type difficulties, and (4) modifying or providing evidence in support of the construct map. We present an example focused on students' proportional reasoning based on its centrality to mathematical success. The results further inform our construct map for students' proportional reasoning and provide a diagnostic item-type framework.