I'm on AQA (I'm assuming you are too because each bit is worth the same as you) and I got an A* and an A in both of my control assessments. For a B overall, I need 13% on the exam (ridiculous I know). For an A I need 46%, and for an A* I need 77%.
I'm not sure what the grade boundaries for the exam on its own are, but I would imagine that if you got a B on the exam you would get an A overall. (Given that even if I get 0 marks on the exam I still get a C overall :P )
Edit: just realised you're on OCR, I don't think that what I said is wrong though because it's similar to AQA