We introduce one instance of a game-based assessment designed to measure students' self-regulated learning choices. We describe our overarching measurement strategy and we present "Posterlet", an assessment game in which students design posters and learn graphic design principles from feedback. We designed "Posterlet" to assess children's choices to seek informative negative feedback and to revise their work. Middle-school students from New York and Illinois played "Posterlet" and then took a post-test, for an overall average of 17 minutes of interaction time. Results showed that the frequency of choosing negative feedback and revision correlated with learning graphic design principles from the game. Seeking negative feedback, but less so revision, further correlated with standardized achievement tests of reading and mathematics. Our research presents a first-of-kind behavioural measure of students' feedback and revision choices and their relations to learning. Within the design context of creating posters, we found correlational evidence that seeking negative feedback and revising are good behaviours for self-regulated learning, and we devised a way to measure these behaviours. This sets the stage for developing and evaluating models of self-regulated learning instruction that help students choose to seek feedback effectively and revise accordingly.