Harman XXV OAK Setting

I am installing the OAK for my new Harman XXV. The OAK manual says to set the OAK opening to 2 for the XXV, then cut the insulation. Everything seems ok, however, before I make the final cut of the insulation, I just want to double check. It seems that changing the setting from one number to any other does not reduce or increase the actual opening in anyway. Is it just my imagination? I rotate the adjustable setting back and forth from one different setting to the other there seems to be no change. What is the actual difference between setting 2 and 7? Can anyone please enlighten me before I make the final cut? Thank you so much.

Yes I did.

Oh, i get. Its not the diameter that is being adjusted by setting to a certain number, but rather, its the height og the intake in relation to the the back of the stove. Gee, who knew! No where in the manual does it ever mention anything like that. Exactly then, how critical is this height relationship and why. Can someone please explain. Thank you.