If the recommended grade is B-, should I add content until it reaches B- or A++?
No matter what the recommended grade is, we recommend that you always add content until you reach an A++. If you have covered the topic comprehensively and looked at the 3 posts on the report (Highest Graded Content in SERPs) for ideas and also covered all those topics, and are still not at an A++, then it's okay to stop.