Expression Var

Hi

How do I write an expression to say "(IF a field is dom air and previous period MINUS if field is dom air and current period)DEVIDED BY if field is dom air and current period " - this is to specify variance  thus  (Previous Dom Air Period Amount - Current Period Dom Air Amount)/Current Period Dom Air Amount  --- to get variance % in a table / text box -- OR any other way I can get this % to work please ?

"Previous"ANDFields!Product_Desc_Review.Value = "Dom Air",SUM(CDBL(Fields!Total_Amount__Excl_VAT_.Value)) - IIF(Fields!PeriodName.Value = "Current"ANDFields!Product_Desc_Review.Value = "Dom Air",SUM(CDBL(Fields!Total_Amount__Excl_VAT_.Value)) /IIF(Fields!PeriodName.Value = "Current"ANDFields!Product_Desc_Review.Value = "Dom Air",SUM(CDBL(Fields!Total_Amount__Excl_VAT_.Value),0)

I also have

"Dom Air",(CDBL(Fields!Previous_Amount.Value)-CDBL(Fields!Current_Amount.Value))/CDBL(Fields!Current_Amount.Value),0))