You can't get an A* at AS. In my school you need at least 90% to be predicted an A* for A2. For Maths at A2 you have to get an average of 90% UMS in C3 and C4. So you need 180 UMS out of 200 in these two units AND you need to get an 80% average over the whole ALevel ie to be safe you want to get an A at AS