Tahj; I don't have practical experience with sizing the gap between the top of the heater throat and the steel heat diverter. I would assume several factors come into play, not the least being the size of your firebox door opening, flue run and chimney cross-sectional size, and how much heat you wish to extract from the flue stream via the barrel top. One video I watched recently suggested 1 1/2" would put the majority of the heat dump directly at the top of the barrel, while increasing the gap to three inches might create a hot-spot torus half-way down the outside of the inner throat.
My question is, if one does not wish to extract immediate heat, but rather send all the heat produced to thermal storage, why bother with the throat?
Here's a link to an article that appeared in Fine Homebuilding, Jan. 87, explaining a stove I built in Missoula.
http://www.finehomebuilding.com/how-to-articles/kang-masonry-stove.aspx?ac=ts&ra=fp
-rockpicker