Thanks for the replies and advice everyone. Jack, yes, as you've noticed, I'm obsessed with trying to get brisket flats to come out "just right."
After my last dry brisket, I realized that I had better success with my briskets before I learned the "probe through butter" test for doneness. As you suggested, I'm going to do some tests at various temps and find out what level of resistance is ideal, and which internal temp offers the best balance of moistness and tenderness.