I could be wrong, but don't believe that the rub is going to do anything to keep the meat from drying out. I believe that has more to do with the cooking technique than anything else. I like to foil my briskets around 160 to 165 internal until they're tender. I put some liquid in the foil to help the process and to add some flavor, usually some sauce thinned with apple juice. Then they rest in a dry cooler for 2-6 hrs. They always come out tender and almost always juicy. Brisket tends to dry out real quick after it's sliced, so I like to paint it with a little sauce too.
As far as the technique and timing,
Juicy and tough = undercooked
Tender and juicy = just right
Tender and dry = overcooked
Falling apart = way overcooked
Brisket can be tough to get right. If it isn't perfect the first time, make adjustments for the next time.
Backyard All-Stars BBQ Team
"Too much pork for just one fork" - SCOTS