I am new for SharePoint less than one month. Yesterday, when I tried to get a calculated field value from the SPListItem, I always got an original value like float;#16.0000000. After one day headache, I get an idea from SDK, the SPListItemCollection has a method named GetDataTable, this method like an interpreter who translate SPFieldType into .net data type natively. so I can get the true value of the calculate field value easily.