I usually keep my exhaust wide open and regulate the air intake. The damper on my verticle intake is usually not opened more than 3/8", which I think has about the same amount of air flow as one bottom mounted ball valve, however by having the larger diameter pipe I have the capability to open up more if needed, same principal as opening more bottom vents.
I placed my intake tube height at about 2" below the top of the barrel, this allows the exhaust to be higher than the intake opening (espically if using holes drilled in the lid). This helps create the draft effect, get the intake to high and you may effect the inlet air flow since you will have the hot air rising and passing the intake opening this may effect the cold air entering the intake tube. If this creates a turblance around the intake opening you may not get sufficent 'cold or heavy' air to enter the smoking chamber.
Keep the good eats smokin'