The accuracy of students’ relative comprehension judgments when reading texts is typically rather low. This has been ascribed to students grounding their comprehension judgments on cues that are not diagnostic of their actual comprehension level. Asking students to complete causal diagrams—a diagramming scaffold—before judging comprehension has proved effective in providing them with more diagnostic cues and thereby fostered metacomprehension accuracy and self-regulated learning. However, there is still room for improvement. We investigated experimentally whether adding the instruction to students to self-assess their causal diagrams: (1) would lead to more accurate judgments than comprehension judgments, (2) would boost their utilization of diagnostic diagram cues by increasing the saliency of those cues, and (3) would enhance metacomprehension accuracy. Participants (N = 427 secondary students in The Netherlands) were randomly assigned to one of three conditions, namely (1) only diagram completion, (2) diagram completion plus diagram self-assessment, or a (3) filler task after reading (control). Self-assessments were more accurate than comprehension judgments, while both correlated strongly. However, no significant differences were found between diagramming conditions concerning diagram cue utilization and metacomprehension accuracy. Apparently, students self-assess their diagrams even without instruction to do so. Nonetheless, the effect of the diagramming scaffold for improving relative metacomprehension accuracy was replicated and extended to absolute metacomprehension accuracy.