Yep, i was shocked too, throughout the WHOLE gcse period i was confident i'd get an A*, cos i thought it was 80% for it and i got 87% in my mock .... so i was like, yeah, i'll get it, should do.
But i ended up getting 92% in the real exam, so not much into an a*. I don't get why it was so high
I was actually so shocked. But then again this is just one exam board, you may not be doing the same one, also remember the boundaries do go up and down according to the difficulty of the paper.
Maths watch is a CD that has like clips of everything you need to know, its brilliant, go on the website and see how u can get hold of one.
If u can't get one, i'd advise just working your way through the topics (in your own time).
If you're decent at maths, and start working through now, theres every chance you'll get an A*. I think a lot of people overestimate how hard it is TBH. Maths gcse is pretty straightforward once you just learn everything you need to know. I know, easier said then done. But its really not particularly difficult, just make sure you know it ALL. Basic rules & formulas. You have ages to go through the course plenty of times, but don't leave it too late (and this applies for all of your subjects).
Having a tutor helps too, in my opinion.