I removed the 2700 degree Fiber Board and the Ceramic Blanket (lays on Board) to clean the chimney. I didn't make any note of the position - which I assume were correct. The owner manual is also silent on the exact placement fore/aft. If I push the Board/Blanket all the way back it will restrict the combustion flow up that route, If I pull it forward it will reduce the frontal escape for combustion flow, but not block it. I'd guess the position should be about in the middle fore/aft.