Dell Compellent Scv3020 looking for a specific OID which gives me the free space which is equal to Available Space - Used Space...

I am looking for a specific OID which gives me the free space which is equal to Available Space - Used Space..When i do an snmp walk using the MIB i get the free space by Available Space - Allocated Space.. But on DSM home page the free space which is displaying is Available Space - Used Space and i am looking for an OID which gives me this value. can anyone help me with this?