Real Value of Used Capacity After Reduction at Storage Group Level for PMAX

Reports Library = > Dell VMAX/PowerMax => Inventory => Capacity Report VMAX.  But when we calculate using it, it doesn't match exactly with the overall what Array is giving.  Like, for instance 'Effective Capacity Used' array wide as seen in UniSphere is 84%, and Usable Total =244.52TB. So, we get 205TB. But, the calculation from above SRM Report gives it to be 212TB. So, there is discrepancy.
So, how can we calculate 'Real Usable Capacity' Total, and at each Storage Group level.
Thanks