You can adjust both, but you are right I don't think you ever want to totally close the top. I just used my kettle for chicken and I had the bottom almost closed (1/8" - 1/4") and the top open about 1/3 to run at 350°. If your kettle does not have a thermometer in the lid you can buy a cheap dial probe type
and drop it through one of the top vent holes. Walmart usually has these in the kitchen gadget area too.