I like this a lot, except I'd be cautious about using "good" there. I know some anxious students who would feel like they were automatically bad at math because they couldn't do all that yet. Maybe "I have mastered the math concept when.
A great article to remind you how to do the hardest thing ever. This also has links to several other articles, including checklists for classroom management and how to handle bad behavior the first two weeks of school.