For me it depends on crankcase volume.
On our Duratec vehicles, they hold nearly 6 quarts of oil, so I don't feel so bad letting them get close to 1 quart low.
My 94 Geo Prizm has a 3.2 quart capacity, so it's closer to 1/4 or 1/2 quart before I top up the oil.
So roughly speaking when 10% of the oil volume is gone, I top it up.
Of course it also depends on how close we are to an oil change. If I'm going to change it this weekend and no major trips scheduled, I'll probably let it go. Conversely, if it is going to be a while, gotta lotta highway miles to drive (especially on my wifes van that gets driven in town almost exclusively,) I'll probably top it up.
[ October 02, 2003, 10:00 PM: Message edited by: javacontour ]