As usual,follow Max' advice.
I'd use an alternative therm to check it as it cooks,and turn dial to around 225* on my alternative therm and work with that.
Your cooker cooks moist, and probably doesn't cook much past the 250* and will probably do it's usual fine job.
If all else fails,every hr poke the brisket with a two tined meat fork,until if feels done.
That cooker was one of the best brisket cookers for a half century,before anyone heard of a therm.
Enjoy the finished product.