Hi there and welcome!
As stated above. Get and use reliable meat probe thermometer. Put the probe(s) into the Thickest yet center most portion of Flat of the brisket... not the point! I always recommend a wireless dual probe (or more probes) thermometer like the ThermoPro TP20 but there are many out there that will do the trick.
IMPORTANT: A brisket is done when it is tender. The Internal Temp (IT) of the brisket is going to tell you WHEN to check for tenderness. You check for tenderness by stabbing ALL OVER with a toothpic, skewer, etc. and when it goes in and out ALL OVER like butter then it is done! It is often recommended to check for tenderness at an IT of 195F and keep checking every couple of degrees or so. If you have your temp probe in a correct spot (which is difficult to do) then often around 201-204F'ish the brisket will finally tell you that it is tender. BTW it takes me 3 probes into the flat to get a good reliable placement for a Brisket temp I can rely on :)
As for what smoker temp to use that is your preference. Briskets can handle from smoker temps 225F - 400F. I do mine at 275F because I don't like waiting for days to cook and eat my brisket and I like it to be done more quickly to avoid losing as much moisture as it may lose over a longer lower temp smoke.
Finally don't be afraid to trim your brisket so that the thin portions are removed and don't burn up while cooking. You can throw that good meat in later to cook if you like or save it for something else. See the following image.
I hope this info helps :)
