You'll have to try a couple sizes out. Also it depends on the dia of your coal basket. In my bullet which is the same dia as your 30 gal drum. I found if I went to anything over 14 inch in dia. I used 2 times the coal to get the temp up for birds. Also a lot more for the 225 temps. Just a thought.
"Good judgment comes from experience, and a lot of that comes from bad judgment." -- Will Rogers