The SSD already sets aside a given amount of space, so you don't "need" to set any extra space. If you do want to assign extra space, then I think you need to use the Samsung Magician software for over-provisioning, which takes unpartitioned space away. Unless you are constant using the SSD, there should be plenty of idle time for the default provisioning to be used efficiently. I don't have any noticeable slowdowns with my Samsung 830s, which have been filled up may times over.