I have never checked the point for temp,I have never dried any out there is to much fat running through it, please don't pay any attention to the deckle/point/moist/fatty end just try not to burn it, as for the flat feeling like butter well I think your just going to have to cook a few more to get the feel of butter(or start checking at 200*). You want to catch the butter feeling on the way up(almost buttery feeling) meat will carry over up to 10* when you pull it. Probe in the thickest part of the flat for checking tenderness right into the top of the flat. I start the cook at 275* after a hour n half I ramp it up to 325* after another hour n half I wrap it and continue to cook until flat is tender about 2 to 3 hours. I have taken every experiment/test run and came up with this recipe.
Here is a older video I did that is tried and true, I just don't cook at 350* anymore unless I'm in a real rush or its chicken, good luck on your future paper bag cookery I see in your future of brisket cooks