The main reason to have reserved space is to allow root to use the mounted filesystem if it becomes filled by an user. Normal users could not write to the reserved space. However, erasing a file (make a copy to some other filesystem) will immediately provide some usable space.
It may become quite difficult even for root to erase a file if there is no free space. I strongly suggest to keep at least a 1% (but never less than 10 megabytes) of free space.
The command should look similar to:
mkfs.ext4 -Eroot_owner=0:0,discard -m1
Probably executed as root. In any case, make sure the correct high-privileges user is set as the root_owner (may default to the user who creates the filesystem).
If you are absolutely sure that you will never make more than 150,000 files, you can also set the number of inodes (if free space is really that important, I would suggest to not use this option):
mkfs.ext4 -Eroot_owner=0:0,discard -m1 -N200000 /dev/sdZ2