i'm new to pumping (since oct '07) and had to make a couple of changes to my I:C ratio recently as i had some strange readings post breakfast and lunch.
i was using 1:10 and was having lows before lunch and also late-afternoon/early evening. i ended up reducing both my basal rates and my I:C ratio, so now i use 1:15 for breakfast and lunch and 1:10 for dinner, i use 1:10 for any snacks in the afternoon...seems to work anyways
the 500 rule was fairly acurate for me when i was MDI.