ISO: Brinkman Charcoal Question?

Got the Brinkman Charcoal gourmet smoker. Tried a three pound Brisket this past weekend as my first test. Planned on it taking 1 hour to 1.5 hours per pound, so I was expecting 3-4 hours. However, after 2 hours my thermometer was reading 160. I thought I had it in a hot spot and decided to leave it on a good 3 hours. Got the nice crispy outside from my rub, but inside was dry and tough - cooked too long. I am thinking my heat source (charcoal below) was too hot. My dial on the lid read "Ideal" when the brisket went on, and when it dipped down to "warm", I added some more charcoal. Probably should of left it alone or not added the brisket until the grill was "warm" - any suggestions on how much charcoal to start with? I used almost 7-8 pounds to start. I should also mention that it was on the top grate. Should I maybe cook a brisket on the lower grate?