There was a thread a while back about where to monitor temp, and someone mentioned that the top vent temp would be most consistent and reliable. Lid temp is affected by wind and ambient temp and grate temp is dependent upon placement. It was argued that the air temp as it exits the top vent is the "real" cooking temp. When I first got my WSM 10 years ago, it didn't have a lid thermometer, so I hung a candy thermometer in the vent, clipped to the vent tab. I got good results relying on that temp. I installed a retrofit lid thermometer and it was all over the place and as I chased the temp, I ruined some cooks. I now use the old candy thermometer and a Maverick on the grate about 2" from the edge and as far from the meat as I can get it. Haven't ruined a cook since.