Reserved % for high redundancy disk group in exadata

Hi all

So we have a question on the reserved space required for rebalancing in exadata high redundancy disk group
For the sake of simplicy lets use the following example

10 physical disks @ 10 TB each = 120 TB
12 grid disks carved out @ 8 TB each to create DATA disk group size 72 TB / 3 for high redundancy = total usable 24 TB
12 grid disks carved out @ 2 TB each to create RECO disk group size 24 TB / 3 for high redundancy = total usable 8 TB

Now we are supposed to reserve 15% space for rebalancing operation otherwise if one or two disk went bad and we replace those but we have used more than 85% of the space