Hmm...that sounds like a 20" wide (having bottom draft controls). Mine just has one on the top, none on bottom. I'm not sure if the burner housing is the same as mine. I have a non-adjustable bottom (burner housing).
I'll just think to the keyboard here: if yours will create a good draft without opening the bottom, or, when they are nearly closed up tight...try that...close the bottom tight, run the top at about 1/4 open and just put a high flame to it and see how she acts. Mine will tell me when she's happy...I get a rumbling sound if the draft is too much, then she gets a bit more quite as the I close the draft down. It depends alot on the wind. If it is calm, I might have to run about 1/2 to 2/3 open on top....windy, she wants to shiver-up and run a little tighter on the draft. Main thing I learned is to listen to the burner...rumbling is too much draft. Open it up until you hear it, and then back it down so it gets quiter.
Try that...also, watch temps for a couple minutes with each adjustment of the draft. You may find that the lower vents can be run mostly closed...I'm not sure on that.
This reminds me of my first couple times of firing up my GOSM about 4 or 5 years ago...they can be an adventure, but once you find out what she likes, it can make for a remarkable relationship. Figure out what makes her purr, and the rest will be history...
Your next questions will probably be how to get the thin blue smoke started, and then, how to keep it going, then, how long it should last...Heh-heh...(been there, too).