Excel.PivotTable pt = xlSheet.PivotTables("MFRData");
Excel.PivotField ptField
Excel.CalculatedFields cfField = pt.CalculatedFields();
ptField = cfField .Add("calc", "='order_qty' * +'order_qty' * 10%", true);
pt.AddDataField(ptField , " calc", Excel.XlConsolidationFunction.xlSum);