Each paper is marked out of 60 (even the synoptic).
However, these marks are then subject to a formula that converts them into UMS (each half of the A-level is equal - 300 UMS in total) so the complete A-level = 600 UMS and the grade boundaries are set in stone against this (can't remember off-hand what they are but you should have been given this information somewhere by the exam board or your school / college.
This is all you really need to know because the rest is out of your hands - when the Board do the UMS conversion from your raw exam marks they factor in a number of things (such as how well or how badly everyone did on a question, the first number they thought of etc.). They then add the 1% or so they're allowed to add by the government just to make sure the overall pass rate continues to improve and, Hey Presto, you have your exam grade.
And if you've followed all this just think how easy the actual exam's going to be.