Ok the opening at top should always be left fully open and the open door by degrees to achieve temp. You do not need much of a gap for temp to rise. It is about adjusting flow. On Weber Smokey mountain. Which has three air vents I use a pencil on all three to start with. That gives me a base point the I adjust to achieve target temp range. Typical 110 to 120c. Try setting it up. The work out a base point from doors for future ref and adjust from there. Base point is not always same of course due to Atmospheric cconditions and is sun shining on BBQ for instance.