Iím not sure that a case can be made where a SSD drive needs to have more free space than is needed for regular use.
With spindle drives, the reason to leave free space is to have enough free space so that the read/write heads can write any file in a contiguous space. Doing this means that the r/w heads donít have to seek where the bits of a file are located throughout the drive. This can take a lot of time. When the r/w heads do have to seek a lot, that dramatically slows down the process of writing to the drive, or reading from it, for that matter. The issue is the mechanical overhead required to find the data on the storage media.
In addition, if one defragments a drive, there needs to be enough free space on the drive so that the largest file on the drive can be written in a contiguous space. The largest file is often the system page file, or the file(s) used by a virtual machine, if one uses VMs.
With SSD drives, there is very little time used for seeking the location of data, because there is no mechanical platter to have to search. Estimates of SSD seek times are about 1 ms or less. From my reading there are no manufacturers of SSD drives that recommend defragmenting their drives as it is a pointless exercise with SSD drives.
SoÖ.leave enough space so you can write the largest file you are going to use and donít worry about it otherwise.