What time would you...

I agree with Chubbs that there is no reason to plan for a really long FTC and just makes the timing more complicated.  FTC is not a necessary step but a great tool to have to hold the brisket if it finishes early.  

Timing a brisket is tricky business.  An hour per lb calculation doesn't work great because an 18 lb brisket might just be longer than a 12 lb brisket and not much thicker.   It might help if you show a pic of the brisket with something for scale and babie some of the pros will chime in.  I would imagine you might trim off 2 lbs.  At 16 lbs the estimate is 1.5 per lb which is 24 hours.  I really don't think it will take that long.  My wild guess would be 16 hours.  I would put it on at midnight (17 hours) and keep the temp as low as I feel comfortable, then adjust as needed the next day.  You can always use foil and crank up the heat if you get in a time crunch. 

I put mine on at 10-11 PM - just whenever I get the fire stabilized and the meat ready, etc.  That's about what time I go to bed.  I try to be below 250 for the nighttime portion of the cook.  Then I normally wake up about 6 AM and check it.  Based on the temp at that time I can adjust the airflow/temp to meet my needs.  Usually that means wrapping in butcher paper and allowing the temp to rise to 275-290.  I can usually get them finished between noon and 3 PM and into the FTC for a minimum of a couple of hours.

This plan is based on my priority of getting a good night of sleep while cooking a great brisket.  By starting right at bedtime and cooking at a lower temp, I feel that I'm cooking the brisket as little as possible during the unmonitored time and I can make whatever adjustments I need in the morning.

Plan between 1h to 1:15h per pound. Each brisket is different so it might be done in 18 hours or 22.5h for an 18 lbs brisket.