Daniel,
Here are a couple of tips that have helped me keep temps regulated when tending the WSM.
First, keep the lid level when you remove it. This helps to keep the heat trapped in the dome.
Do not add water unless you need to. Chicken is generally a short cook so you shouldn't need to add water. In fact, you shouldn't need water at all for chicken, but that's a different matter.
Similarly, add only as much water as you need. There's no need to fill the pan to the brim when there's only an hour left to cook. OTOH, fill the pan at bedtime when you're doing an overnighter. Also, I usually add water through the access hatch between bastings. This helps avoid heat loss.
Plan ahead. Try to understand where the temp is headed. To me, that's as important as the current temperature. If you're going to do something that will cause heat loss, consider running the temperature up a little before. Be prepared to watch temps after and make adjustment accordingly.
Last of all, feel free to make adjustments to the vents. Conditions change: wind, sunlight, coals burn down, etc. I find that adjustments take about 10-15 minutes to be noticeable. Temps usually recover with 10-15 minutes afer removing the lid. Watch your thermometer and make adjustments if needed.
Tony