while disk space doesn't have a direct influence ... lots of little files (as chuck says - like photos - and ironically also like windows) cause havoc because the database of what's where (mft) is so big and it also increases the potential for fragmentation as you add a few here and a few there
(having more than 25000 files in a single folder is another good way of messing with NTFS)
like many things in windows the ntfs file system (in theory at least) uses a proportion of what's available (like the waste bin and system resore) - MS say it needs a 20 -25% "overhead".
( if you are interested read
http://support.microsoft.com/KB/315688
but it's pretty boring)
so you are absolutely right in quoting the %
however the total is also important..
CCleaner will help keep the count down
http://www.ccleaner.com/download
and a defrag every so often won't go amiss either .... but the best solution is get a CD/DVD burner .... or a USB HDD and shift as much data as you can over to it