While I think the post by Bigabyte, liked by gtr, is a great pictorial and narrative on how to do a brisket. The one or two things I notice.
1. I really like a packer brisket in the 12 to 14 lb range, I think they are more forgiving and the flat is a little thicker, hence more forgiving.
2. I like to cook at 250F to 270F for briskets. But, 225F would be my minimum.
3. You did not say how you are monitoring your meat in the cooker. I like to use a thermometer on a cable so I can see internal temps of meat with door closed. It will normally take 10 hours to get to 190F.
4. Once it hits 190F, I use a metal skewer, some use ice picks or thermapen probes..and I check for whether it is done by poking, when the probe goes in easily, it is done. I prove across the grain slightly and in two or three spots in the flat.
I'm feeling bearish, and I'm packing a Wusthof Grand Brisket slicer from MABA
"perhaps...but then again...maybe not..."