For more accurate measurement of the temp near the meat/grate rather than an oven thermometer, you could use a confection/deepfry thermometer, since it has a probe that sticks down 4 inches or so? So if you place it in the top vent, the tip of the thermometer is close to where the meat is, and you won't have to do any calculations of how much hotter or cooler it might be right at the lid.
Makes sense?
Makes sense?