In Reporting Services paginated reports, expressions are generally used to control content and report appearance.
IIF is a decision function that returns one of two values depending on whether the expression is true or not.
I hope this would help you:
=IIf(IsNothing(Fields!field1.Value)Or IsNothing(Fields!field2.Value) ,0,((Fields!field1.Value)/(Fields!field2.Value)*100))
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