This is my second build and i figured out all the stuff i didnt want to do on the first one so here we go again, went from 320 to 500gal, single to tandem trailer so i have room for all accessories and storage on the trailer, dual racks instead of single. Racks are in, doors etc. my biggest mistake on the first was the firebox, i used a fuel tank from an old oil furnace, super thin metal, this time im buying 1/4 in plate, so here is my plan for this one. feldon bbq calculator says my firebox should be 38500 and this design is 38800 36wx36lx30h so almost dead on. i need help figuring out how high to put my RF plate so that the throat from FB to tank is big enough. and the calculator says i need 152 chimney if i go with 4 in, which seems really tall. even with dual 4in pipes saying about 7 ft tall, how much shorter can i go if i put an opening between the tank and warming box and have the chimney come off the warming box?