A levels are marked out of 600 UMS.
Grade A requires 80%, so 480 UMS.
Grade B requires 70%, so 420 UMS.
Grade C requires 60%, so 360 UMS.
Grade D requires 50%, so 300 UMS.
Grade E requires 40%, so 240 UMS.
Raw marks to UMS marks are calculated by giving a certain percentage of candidates a 'C' grade. One exam is likely to be marked out of anything between 90 and 120 UMS. As an example, assume an exam is marked out of 100 UMS. If you get 80% of the exam right, and it is a hard exam, your UMS will be greater than 80 and you'll get an A for that paper. If you get 80% of the exam right, and it is an easy exam, your UMS will be lower than 80 and you'll get a B for that paper.
Does this help?