This is the thinking they used. See the blog post today on the Pure site about DirectFlash Enabling Software And Flash To Speak Directly and you'll see an example with the Purity/RAID 3D overhead worked in on a 9.1 TB DirectFlash Module. Usable from that is 5.23TB. So for 18.3TB DFM you get over 10TB usable. Their global average dedupe rate, which they publish live on the site under the Products-Purity section, is ~5:1. There's 20 DFMs. 10x20x5=1000.