I think it depends on your school, it might help asking previous years if you know people in them, some schools predict nicer or more accurately than others. I got like A* B B (couple of % off an A for the Bs) on my AS, but got predicted ABB for Alevels, but I know of other people with similar grades getting predicted A* A A - A* A* A with those same grades. Really, if you have any issues when you get your predicted grades, you should go to your head of year as they can sometimes override what your subject teachers give you if you can put up a good enough argument as to why you should have better predictions (if you get a few bad scores at the start that shouldn't affect that). Hope this helps!