i have no idea about that , but in most A Level syllabuses ,
A candidate who achieves
… the minimum mark necessary for a Grade A obtains a percentage uniform mark of 80
that does not mean you need 80% of the total marks to get an A
it means the marks set by cambridge for the achievement of grade A with minimum marks is equal to 80%
and a 100% isn't a 100% .. you're bound to make mistakes. CIE is tough.
it means you got more than the minimum marks required for A* but those marks map onto 100%
my guess is, anything above 170/200 was equal to that in my exam but i'm sure for Math it was around 185/200.