Also i don't think you can use Aggregation functions with column references!!
as you said A and B are expressions which means they have aggregations functions like sum,min or max
so Sum(A) or Sum(B) will never work!
To have Expression totals you need an aggregation function so i suggest you rewrite the expression for A and B
C = Full expression for B / Full expression for A * 100
Vineeth Pujari
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