First, let me start off by telling you that I am BLONDE and have no affinity for anything involving numbers. That said, we are ready to pour our insulating hearth. I have read the Pompeii instructions over and over, still unable to find the answer to my question. I have scoured the forum and still have come up short. The issue that I have is how does one calculate the amount of portland cement and vermiculite......I understand that it is a 6:1 ration, no problem, I understand that in cooking terms. But how does one know "how much" will be needed? The island hearth for the insulating slab is 50" X 50" by 3.5".....I have plugged it in to the calculator and gotten numbers for cu ft, cu in, but don't know where to go to further the equation......I don't want to start the pour and find out that I need more of everything!!!!! I am 45 min away from the nearest Lowes. HELP!!!