ok, so on your coursework you got 16% overall (calculated above)
16%/20%
approximate grade boundaries;
A* 80%
A 60%
B 40%
C 18%
so, you already have 16% (80% if marked /100% for the coursework itself)
therefore, if the exam is out of 100%, then 100% on exam = 80% overall, (multiply by 0.8)
Therefore you will need 80% - 16% = 64% overall
64% / 0.8 = 80% minimum on the exam paper for an A*. Does that seem reasonable??