How much free space should be in my C drive?

Now I have 30 GB filled in my 80 GB C: drive.. My laptop has 3 GB installed memory and Win 8.1 x64 OS.. So, I've allocated 3 GB for PageFile, 2.25 GB (by default) for HiberFile (which is 75% of my RAM) and 5 GB for System Restore files.. These three together already consumed 5 GB I think as opening C: drive--->Ctrl+A-->properties show 25.3 GB.. Assuming that I don't install anymore programs/games in my C: drive.. So how much free space (in % or in numbers) should I allow as free in my C: drive so as not to affect speed/performance?