Sounds like a good plan to me. Flats can be tricky, since they are the leaner part of the brisket. So the things I really like about your plan is the injection, foiling at 165, and a good long FTC rest. The only thing I'm wondering about is whether you might have started just a tiny bit early. Briskets usually take 1 to 1.5 hours per pound at 225. Since you are going 220, probably closer to 1.5 hours per pound, or about 12 hours, maybe less. If it is done before noon, you can FTC for several hours, even up to 5 hours or so. If it's more than that, you can always put it back into the smoker at 140 (allow the smoker to cool down after the cook before you reset the temp to 140) to keep it warm for a very long time. Better to have it done early, than not done in time and pulling it too soon without a rest.