If no-foil, straight open grates throughout the cooking, you're pretty close, but I'd figure 18 hours for the brisket, as it has less cross-sectional density than the butt, overall. I've had 8.5-9.0lb butts and 16-18lb briskets run over 25 hours...depends on the smoker, stability of chamber temps, chamber humidity, etc...and every piece of meat seems to have it's own personality...some are faster than others. Also, bumping temps up to around 240* after 4-6 hours won't hurt, either...picks up the pace a bit, but it's still slow enough to get decent tenderness...well with a smaller brisket like you have, 240* might be a bit too fast...I was thinking more about monster butts and briskets there.
Before you yank 'em out to rest, poke around inside a bit to determine the actual tenderness...don't worry about loosing juices from pokin' it a bit towards the end of cooking, just don't stick it 50 times...LOL!!! The brisket flat will cook faster than the point, so probe both ends of the brisket for internals...you can always separate the point/flat when the flat reaches a tender state and continue cooking the point as needed while the flat is wrapped and resting. A bone twist with the butt should indicate the state of tenderness...if it feels like it's ready to pop out, it's done.
Most importantly, have a GREAT smoke, and enjoy the day with your friends and family!!!
Eric
