I know this is probably the stupidest question ever posted here but here goes: I am about to pour the insulating floor of my oven, the renforced floor is done and the perlite is awaiting my attention. On page 22 the verbal instructions indicate the floor needs to be upon at least 3 3/4 inches of insulating concrete. The photo, on the other hand, show the floor bricks flush within the 7 1/2 in forms. 3 1/2 inches for the support structure leaves only four inches left till the top of the form. Do they mean the overall thickness of the insulating layer is four inches and the floor firebricks are still insulated sufficiently on the bottom?