When evaluating Ceph to run a new storage service, the replication factor only matters after the hardware provisionned from the start is almost full. It may happen months after the first user starts to store data. In the meantime a new storage backend ( erasure encoded ) reducing up to 50% of the hardware requirements is being developped in Ceph.
It does not matters to save disk from the beginning : it is not used anyway. The question is to figure out when the erasure encoded will be ready to double the usage value of the storage already in place.