this being said if you do not know where fits your needs best then as I always say, make it adjustable...
I tune the pits I design to cook more evenly at lower temps.
a good rule of thumb to start with is:
- you want the gap to be bigger in square inches than the area of your smoke stack opening... 6" diam. round smoke stack opening= 28.26"sq. so your gap area would need to at least that big.
- you want your smaller in square inches than your firebox to cook chamber opening.....