Greg,
Here's how I do my briskets.
Cook with a dome temp in the 245-260* range fat cap up until the brisket hits 165*. If you are cooking a flat, foil and continue to cook until the brisket hits 190*. At that point check in several areas for tenderness with a meat probe, the probe should slide in with very little resistance. If there is resistance, continue to cook until you can easily slide the meat probe in, in several spots of the brisket.
If you are cooking a packer, cook as stated above, but remove the point from the flat before foiling the flat. The point will require several more hours of cooking to render the fat and to get tender compared to the flat.
Or what I like to do often is to remove the point and cook the point and flat separately, that's just a personal preference.