I think this is a hard one to accomplish, as the style of cookings you can do on a kettle can have varying temperatures at varying locations. If it is for indirect cooking, I would be using the thermometer to ensure I do not go too hot, so I would place it near the exhaust vents, as this should be the hottest place with the exception of right above the coals, but you aren't cooking over there. This would be the best location if I was to install a thermometer. Direct grilling could probably be satisfied with a thermometer anywhere in the dome so once again, near the exhaust vent should be fine. You actually cooking temperature will probably be lower than this number though.
XL Big Green Egg, Weber Performer Platinum, WSM 22.5"