optimum clearance/ size measurements

Hello one and all. I know there are some very good designers out there and so i have already had some information given to me about a new build to extract more heat. In the asking and sharing of information a couple things were shared and this also brought other questions about optimum size/ height/ clearance. I will include a basic drawing I made which is a fairly typical build design if you are using metal. We have a 5" J-burn tube surrounded by two metal barrels, the first or closet one to j burn chamber insulated. Now the questions. In inches what should be the best clearance between J burn and first barrel and then best clearance in inches in between inner and outer barrel. Please excuse my spelling it is terrible. Also I had saw a video which Paul Wheaton I believe said the outer casing barrel could be shorter which of course means the burn chamber is shorter. Is there an optimum height for this and what would be from your research the optimum height clearance between the j bun chamber and the top of outer barrel.