I'm not 100% sure where your figure for 70TB for NetApp Health comes from? In my previous post I tired to explain how much space in the aggregate you can use.
When you create an aggregate, you define the number of disks and how many RAID groups (i.e. how many disks will be used for parity i.e. non-data disks). The 10% WAFL reserve is behind the scenes and not negiotable. You may however use the entire rest of the aggregate space for data, if you so desired - but strongly not recommeneded - see previous post. There is no element of NetApp Health, except the best practice of not trying to exceed 80-90% full because of availablity and performance.
The output of df -A -t will give you the amount of space (in TB) that is available to you for creaitng volumes and making available for data use. You cannot change this amount, it is dictated by the number of disks you have available and your RAID group layout.
Have you considered thin provisioning where you can over allocate the aggregate by creating volumes larger than the actual capacity available. However this comes with it's own warning, that the agrgegate capacity must be constantly monitored and plans in pace to address any lack of space, either purchase new disk or free up space by moving volumes into different aggregates ordeleting old/unused volumes.
Also, storage efficiencies will allow you to store more data in the same space. Depending upon your version of controllers and ONTAP version, you can implement deduplication, compression and compaction. These can offer great savings, especially for virtual environments and CIFS data.
For both thin provisioning and storage efficiencies please see the Logical Storage Management Guide for the version of ONTAP in use - ONTAP 9 link here: https://library.netapp.com/ecm/ecm_download_file/ECMLP2492715
Also, TR-3966: Data Compression and Deduplication Deployment and Implementation Guide for Clustered Data ONTAP: https://www.netapp.com/us/media/tr-3966.pdf
Hopefulyl this helps you maximise the space you have.
Thanks,
Grant.