Well, time can vary pretty widely from brisket to brisket. At 300 degrees I suspect you will get it probe tender quite a bit sooner than 12 hours. It should start to get close to ready to take up somewhere around the 195 internal temperature mark.
When it is probe tender in the flat, take to off, wrap in the butcher paper, and then place in a pre-heated cooler To rest for a minimum of two hours wrapped in towels with all voids in the cooler filled.
At least that is what I would do.
Good luck, relax and enjoy.
Jim - Another transplanted Texan
Former KCBS CBJ
Large and Medium Big Green Eggs , Black 18.5" WSM, Blue Weber Performer - Stainless, Green Weber OTG Kettle , Brinkmann SnP Pro, and a Stainless UDS. One retired Portable Kitchen grill.
Red Thermapen, Maverick ET-732, EdgePro Apex Sharpener.
Avatar is the original 1951 Weber Kettle