To get an A* in most A-Levels, you need a 90% average from your A2 modules and an A grade at AS-Level. Now in some subjects, it's different. For example, in OCR A-Level Maths, you need a 90% average from C3 and C4, only two of three A2 modules you have to do. So you can mess up the other A2 module for that qualification and still get an A*. That's an exception, and there are probably more as well - normally A* is 90% average across the A2 modules