In fact the recommendation is for 15% minimum! So if the HDD is say 200GB, requires 30GB free.
Yeah, not really.
There's no "hard and fast" rule, and certainly nothing official published from Apple about this. People say different amounts, but in point of fact a clear percentage is actually the LEAST sensible of the various yardsticks -- I mean if you buy a 2TB drive, you're supposed to leave up to 300GB free? Puh-lease.
In truth, what people are TRYING to say with these helpful guidelines is simply that the Mac requires a fair amount of temp space in order to work efficiently. How much? Well, again, in truth, that varies based on what you do. Someone who primarily uses their computer to read email, post to Facebook and play Solitaire is not going to see a performance hit if they let it get down as low as the OP has done. People doing "real work" on their computer, however (which I define as having several apps open, using large graphic files or video or audio, power uses like that) will want to take that 15% (or more!) to heart.
I always recommend a dead *minimum* of 12GB available at all times, and that's based primarily on how much I've found the OS needs to burn a full 4.7GB iDVD. Most of us don't do that very often, so 12GB free should be more than enough to handle the other stuff OS X does need temp space for, like virtual memory, on a routine day-to-day basis. But really, more than that is better -- and if you really need a hard percentage, here's one for you: when you get down to say 15-20% free space, that is your queue to start thinking about how to either move off stuff you don't need, or buy a bigger drive (and of course how you're going to back it all up).
Hope that clears things up a bit. It's more long-winded than some imaginary "rule," but it's also more accurate.
