Recommendation: Monitor the IT in the center of the flat. Reason? Usually, the flat is sliced and you want to control the temperature more tightly there to get good slicing texture. The point is fattier and often diced up into "burnt ends" for even more cooking so the temp of the point isn't the critical one.
ITs can be odd at times. Sometimes you get a fairly rapid rise and sometimes you don't. Reasons vary but can be the point at which the lid thermometer or "hood ornament" is sampling the temperature vs. what the grate temp is. Sometimes you get a "stall" where the temp will hang and won't change for a long time.
There's been lots of good advice here so just go with the flow. When you get close to the target temp in the flat you can start poking with a toothpick or skewer -- if it goes in easy and "feels like butter" you're golden. This may occur a few degrees either side of your target because briskets are just mean and ornery that way
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