You should place the probe on top of the heat source but with a small bit of tub between it is a bit more accurate that way.
http://www.youtube.com/watch?v=a2jgk...eature=related
I'd recommend checking the top and bottom. Heat rises and my bottom of the rack is the coldest and the top warmest it is by 5 or 6 degrees. I actually don't use the bottom at all. Besides it is very hard to clean that one anyway.