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It really depends on the temp. If you're going 200-250, then you'll just need to have the daisy wheel cracked a bit. If you're going 300+, then open it a little more. It depends on your charcoal, outside air temp, and lower grate too.
If you see the thing constantly flashing green (low), you probably need to open it a bit more. If it's red (too hot), close the daisy wheel some more.