I don't actually mix a rub, I apply each part to the brisket individually. I pretty much go by eye; I don't have proportions.
I start with the brisket blotted dry with paper towels. I use Kevin's method for the salt, and I do that first. I do a dry brine on turkey, and when I read about Kevin's method of pre-salting instead of adding salt to his rubs it seemed similar in concept. After the salt has had a chance to react with the surface, it will moisten a bit that holds the other spices.
I would describe the kosher salt, black pepper and thyme amounts as enough of each to well cover the surface. (I don't do thyme or powdered chiles on the cap side.) Not opaque or anything close to that, but more than I would on a steak. It may seem pretty heavy handed to use basically as much thyme as salt and pepper, but I like how it adds high notes to the finished brisket, and I want it heavy enough to be there after the cook. The powdered chiles are to taste. Maybe about 1/4 to 1/3 of the amount of black pepper.
I know this is vague, but I'm really going by eye, and I'm having trouble figuring out how to describe it. When I do a butt, I do a traditional rub (again sans salt) and then apply it to the meat. This brisket method is more like how I apply spices when I'm grilling.