Hello @JessGrocott
Thank you for contacting the Instructure Community. You're doing this with graded quizzes? Not surveys, ungraded, assignments, etc. ? Normally if it is for quizzes, it is going to be automatic. If the student completes it (even a single question T/F quiz) it will mark it as complete. If they do not submit the graded quiz it will stay as incomplete. If it is not functioning like this with those settings, you may want to contact Canvas Support directly. There are also others who have been looking for solutions in a similar fashion:
New Quizzes survey workaround and complete/incompl... - Instructure Community
Hopefully this helps, and if you need to contact Canvas Support:
How do I contact Canvas Support? - Instructure Community