How do I calculate an over or under-provisioning amount of space for my SSD?

In my proactive attempt to maintain my SSD’s performance and knowing that it degrades as it fills, a user should benefit from keeping a certain calculable amount or percentage of space “free” (Over- or under-provisioning), correct?

To maximize storage, use and performance, what would I need to do to calculate and configure this space?

Info:

SSD Drive: 120 GB Kingston Hyper X Sata III drive. (111.75 Usable).

TRIM is supported and windows recognizes the drive as an SSD utilizing TRIM.

OS: Windows 7 Home Premium 64 bit service pack 1 (20 or so GB, 91 GB remaining)

Various programs and files (40 GB, 41 GB remaining)

With the remaining 41 or so GB, I believe that I will benefit by locking some remaining available amount to maintain the performance of the drive. How can I calculate the amount so I know how much remaining space I can use for files and programs and how much to secure as free?

Edit:

Most of the activity will be reads and average home user writes to the drive, So I am not as concerned about endurance as I am about performance in regards to allocating the space.

Interesting that one of the advertised features of the HyperX is "User-Configurable Over Provisioning" (http://www.kingston.com/us/ssd/hyperx). I'm not sure how that feature works.

In any case, http://www.anandtech.com/show/6489/ is an article which tests the correlation between SSD IO performance and free area. For the absolute maximum performance, you want to leave maybe ~20-25% of your drive free, depending on the drive. Edit: This drive has 144 Gb total NAND according to this review, so about 108 Gb used ought to do nicely. You could do this either by partitioning (the other answer is incorrect, the drive will wear-level), by using whatever Kingston is advertising (I assume), or just by not using up all of the space on your drive.

SSDs use a process called "wear leveling" whereby they attempt to make each cell use data the same amount of times as all the others, thus preventing some cells from going bad quickly. By reducing the amount a space your computer could use, you may be prematurely aging the active cells by not allowing the leveling to take place on the space you reserved.

In addition, I have read (quick source, detailed source) that drives with non power-of-two sizes (i.e., your 120 instead of 128) have already reserved that excess space for when some of the cells go bad.