[ADMIN] Amount of unused space on a storage device

Hello everyone,

We have an OLAP database on dedicated RAID storage with a total capacity of 100TB. The transaction log and temp_tablespace are stored on a separated drive.
All tables are partitioned by timestamp of insert, the size of each partition is 1 week. All tables are insert only, no update or delete ever occurs. The amount of data stored peer week is ~1TB.

We use following maintenance algorithm:
  IF amount of free space on storage is lower than BOUNDARY, THEN
    backup all partitions for oldest week
    drop these partitions from database

Our desired behavior is to keep as much data as possible in database for analytical queries.

My question is:
Having 100TB capacity and data coming in at a fixed rate of ~1TB per week, what would be an optimal BOUNDARY? Or to put it another way, what is a reasonable amount of unused space on a storage device of this size?

Thanks & Regards,
Michal Merta