Without any holes in the deflector, you are blocking alot of heat from radiating straight up. You will have a lower temp in the center of the drum instead of having one with holes that equalizes the temp closer across the drum. My deflector rack is 18 inches up from the bottom of the drum ( about 6 inches above the coal basket). my first cooking grate is 5 5/8inches ( 6 inch bolts), the second/top grate is again 6in carriage bolts which ends up with around 5 in to the flat top under side. Hope that helps.
"Good judgment comes from experience, and a lot of that comes from bad judgment." -- Will Rogers