VxRail datastore storage

I have one set of VxRail system with 4 ESXi hosts, total VSAN storage space is around 120TB.

currently its one single VSAN storage for all the VM in the 4 hosts.

question, let say i can start from scratch to configure the VSAN storage, can i leave 30TB as unused partition for future use or I must by default select all 120TB as usable storage ?

right now my usage is 25TB out of the 120TB with 85TB free space, which is think is rather wasteful