A tip for today,is plan on them taking longer than you think.
There is no straight line between weight/ cooker temp/internal time =done enough.
Stick your two tined meat fork all the way into the flat and try to pick it all up.
If you can pick it up,without it falling off the fork,it probably needs to cook more.
drbbq used to teach us to cook it until you are sure it is done and then cook a couple hrs more.
Not perfect,but on small briskets, it may get you close.
Be patient,let it set in the foil for 3-4 hrs,you'll be glad.