Experiment. While unscientific, I used a turkey fryer therm and tried to figure it out myself by sticking it in through the top vents and the hatch. In fact, I found that the bottom grate was about 10 degrees hotter than the top grate, with the bowl in place. Most folks tend to echo Dwain since heat slips up the sides and essentially projects from the top downward. Not for me, at least not yet.
I've found that the dome thermometer reads about 50F lower than the temperature at the top grate and the top grate is about 10F cooler than the bottom grate.
...Seriously, I think the temperature difference varies depending on how you treat the water pan...with water, empty, filled with sand, clay plate, steel plate, gold bars, etc.
...The plan is to move the lower grate on my 22 up 3" so I can get an aluminum pan on the lower grate for water during the 1st 4 hours of cooking time on briskets and butts just to add some moisture. I don't use a dome thermometer, not cooking in the dome...
I'm pretty new at competitions...What is MBN?