yikes!
The gods of food safety are gettin very concerned...
Not sure what you mean by half way today and the rest tomorrow. Will you keep it hot throughout or throw it in the fridge in between?
Remember: the danger zone for making good meat very, very bad is when it sits between 40-140 degrees. You don't want it there more than a couple of hours (4 hours by the book), but that includes cooking, holding, chilling, reheating. All the up and down is dangerous.
So cookin it to somewhere in the danger zone, then cooling below 40, then bringing it back up through the zone til it hits 200 is a tad tricky. There are several hours of worry there...
Assume you do not have a CS or else you could put in overnight... start at midnight and when it's done the next morning just "hold" over 140 until dinner time.
Since an 8-lb brisket cooked at 225 will take roughly 1 to 1.5 hrs per lb, you'll need to figure 8-12 hrs to cook it. If you're burnin wood, I would start early in the morning (6) if you want to eat at 6 or 7 at night.