I am to the point that I need to put the exhaust vents in the drum lid. As it sits now, I think that there isn't any reason to make the exhaust adjustable. I am thinking that the amount of air you let in the bottom is going out the top. If you choke of the exhaust, you could override the air intake metering at the bottom. Basically, the air that flows through the system is the lesser of the amount you let through the bottom or top.
Am I thinking correctly? is the general concesus (4) 3/4" diameter holes?