Glenn's response raises an interesting question. If no water makes the bottom grate 20 hotter and water in the pan makes the top 20 hotter, is there a middle ground (say a small water pan within/on a clay pan) that could be tuned to get the same temps top and bottom. The physics suggests there is. I might try this as an experiment if I ever get the time.
Don't over think it. There is some difference in temps throughout the smoker, but remember.. you are cooking at very low temps for hours and hours. Even things like wind and sunlight can affect the temps.
Just put a probe in where ever it's most convienent. I've cooked for years without any probes at all and just a meat thermometer dangling in the dome.
I agree with Mike....in smoking the temp isn't that big of a deal
...what is important is doneness checks! Just use your weber dome therm and keep the needle around the vertical position....any where in the shaded smoke range is close enough!
When cooking roasts use your probe therm as a guideline to perform doneness checks