Your times sound a little short. An 8 pound butt will need to go about 1 1/2 hours per pound at 250?, almost 2 hours per pound at 225?. You want to go to about 190? internal temp on the butt. I'm not sure what to tell you on the brisket flat. I always do the whole briskets and I go to about 180? internal on them, then let them rest. As someone else stated in another thread, the internal temp will rise if you tent it with foil. It's not a bad idea to try to be done 1 to 1 1/2 hour before you want to eat. For one thing, the meat always takes longer than I think it will, for another, I think the rest period tends to make the meat more tender/juicy.
Good luck,
Doug W