After doing the math a number of different times. The configuration you want to do as having 70TB of free space using 3TB drives in a RAID 6, is just not possible as there are only 24 drive bays. With 3TB drives you will get about 2.6TB of free space as you have to count for the header space that will not be accessible and in a RAID 6 configuration you have to include 2 HDD's that are for parity. The most space you can get with 3TB drives will be close to 66TB without counting for the header space from each drive. 
Hi all  We wanted to have 100 TB storage of RAID 6- out of which 70 TB with 3TB sata disks  and 30 TB with 600GB SAS disks. Please clarify how many disks of 3 TB and how many disks of 30 TB is required for usable 100 TB with best performance of RAID 6.  Also please clarify part no: 403-BBDC related to MD3620f dual controller with 4 GB cache each.  Please clarify me ..its urgent  thanks
RAID 6