Temp controll on 640

This is what works for me after a lot of trial and era. I have the heat defuser with the trap door on it. I remove the trap door and put 3 grill grates in. Slide heat shield all the way open to the right. Top shelf temp runs about 30 degrees lower that what I set my pit at., but it stays consistent cooks great . Hope this helps some. Also I only set my startup temp 30 higher than I want to cook at.