"Usable" space is difficult to accurately say, if DeDupe or Thin Provisioning (or both) are being used. Could be as little as %78 of the space provisioned to us (no NetApp features used), or as much as %1000 of the space provisioned to us (large VDI deployments with DeDupe).
Of course, if Snapshots are being used and a true apples-to-apples comparison is made, even our worst case will exceed %100 of what the same storage presented directly to a host would provide. I would encourage you to frame discussions around capacity in those terms. Yes, we will take a little off the top, but we'll pay it forward many times over if you leverage the storage efficiencies DataONTAP can provide.
The "overheads", such as they are, are discussed in detail in section 7.4 of TR-3461 (V-Series Best Practice Guide)
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