I have been building drum smokers for many years.(I think I am on the 4th page of the massive UDS thread) So I do have some experience. I think I have built around 25 smoker so far for myself and friends.
The best design I have to date is 4 -7/8 inch intake holes and 6 equally spaced 7/8 inch exhaust holes on the top. Just seems to work the best. Why 7/8 inch.....size of a 1/2 inch NPT slug cutter.
I will say that I built 4 IDENTICAL drums three years ago for a chicken wing cookoff and all four of them cooked differently for some reason. Not that much different but I had to adjust each one differently during the cookoff.
Just my $0.02. There is no BEST design....just find what works for you and cook enough to "learn about" your drum. The ones I equipped with a fan and PID controller could control temperature on the dot but what fun is that!
Bill Napper. Team Cook "JoeJo's BBQ"
Custom offset stickburner
Gas assisted cabinet smoker (400 lb. capacity!)
55 gal. drum smokers galore! (All custom built)