It would appear I have confused some of you with the drawings of the new design of version 3 (8 inch system) vs the current 6 inch system I'm running.
On the 6 inch platform, the base of the fire-tube is 1 inch of #2 firebrick. (this stuff is supposed to be much more heat resistant than normal firebrick). The brick below that RMH (the same as in the drawing) is simply for insulation (along with the lower levels of dura-board and marble) so I don't toast the carpet underneath.
In the future system (8 inch per the drawing) the layer of firebrick will now become the base of the fire-tube (see new drawing). Again the goal is not to heat the bricks but contain the heat of the fire-tube. I posted the pictures because the RMH guide is just a bit off, because when I added the drum, it appears there will be a conflict between the drum and the feed tube using the specs in the guide.
I've modified the new design to measure the riser from the bottom of the burn tunnel, and that seems to be good advice.
The only remaining question I have now is, I received counsel that I should have the exit from the drum about 4 inches from the bottom of the barrel, I did that on this latest build (5 inch) but that seems pretty high and will cause a lot of excess mass to fill in the box and since heat rises I wonder if I should lower the exit somewhat? Any thoughts? My issue is I'll have to fill in about 6 inches of mass (pebbles in this case) just to reach the bottom of the exit tube. It just seems like a lot, that's all.