The percentages for grades go like this (generally):
A: 80%+
B: 70% - 80%
C: 60% - 70%
D: 50% - 60%
E: 40% - 50%
U: below 40%
This is usually what determines your grade for the subject after your modular UMS has been added together. These grade boundaries don't usually change only the grade boundaries for each exam changes effecting the UMS for that one.
Note: You can also get an A* with 80% average UMS overall in both years and 90% average in A2.