Hi Wilsonk6!
I built a standard sized Scott oven 7 years ago and it is fantastic! I used standard concrete mix for the hearth slab, not refractory cement. I used a thin layer of refractory mixed with sand to weaken it under the hearth bricks. Then used a standard mix of refractory cement in the dome. I think I spent about $300 on the refractory cement for the whole project. Even with the oven cranked up making pizzas, the outside of the firebrick doesn t get above 550 degrees, so I m not too worried about the integrity of the standard concrete. The thickness was 3.5 plus the insulated slab at 3.5 I believe. Add in the thickness of the firebrick, and the whole slab is pretty massive already. I m not sure you would need to go thicker.