I have an 80GB and the math is pretty solid. I have about 35GB free on my 80GB, and I personally wouldn't want to see it drop to less than 10GB free, preferably 15GB (20%). The more free space you have on the drive, the longer it lives, not that it should ever be a problem, I think worst case scenarios show the drives taking decades to be written to death. You also need space handy for installing crap/temporary files, lots of games are sloppy and use your my documents folder to install DLC (which can take up many GBs), not to mention saves, which in games like The Witcher, can also take over 1GB.
To simplify the formula, I find it to be:
Total drive size -5% - 35GB = free space.
80GB - 4GB - 35GB = ~40GB and I have 35GB free (I believe my swap file is 8GB not 4GB, which accounts for some of the difference).
Not wanting to go under 10-15GB leaves me 20-25GB free to use (which I doubt I ever will, at this point my PC is basically thoroughly broken in).
The drawbacks of putting office and photoshop on a 7200rpm are that they benefit from the SSD significantly more than most any game will. If you never/very rarely use them though, putting them on the SSD does very little good I suppose. I only use office like maybe once a month, and I still would never want it on my 7200rpm drive.
More likely I might just be forcing my personal opinions too strongly, and you'll be fine running it with plenty of space, for all I know your entire windows install could take 20GB with swap file and apps and FFXIV could be just 10GBs, leaving half the drive free.