Here is an extremely crude drawing of my layout. The wall to the left is an outside wall, and I want to minimize heat going to it, so I planned for the barrel (gray circle) to be as close to the heat riser (red circle) as possible. Originally plan was for all the smoke to exist from area A, which has an area of 220cm^2, but now I think if I connect also area B, which should have almost the same CSA as area A, then I'll get 400cm^2 total CSA, though an area that would be hard to access for cleaning out.
The footprint of the RMH is 1x2 meters (3.25x6.5feet). Where the yellow circle is will be a point for the smoke to go up, and then back down another pipe horizontally before going up the chimney, that pipe being in the sort of chair-back that will be formed in the bench. Total exhaust flue length will be about 5 meters (16 feet), including turns. Which is kinda short, but I don't really have much room next to the chimney and I didn't want to put the bench along the outside wall.
