Hearth slab question

I am in the process of building a brick oven and have a question on the hearth slab....  I am following plans that say to use calcium aluminate in the mix for the hearth slab, which is supported on the cement block with a grid of rebar.  I have the insulated slab already done underneath it..... I have talked to a guy that has a oven he made and used just regular concrete for his. He says the firebricks above are enough, and it doesn't need any special concrete for it.....  My thought is maybe just pour the regular concrete then after it dries pour a 2 inch layer of refractable castable on top then start building the oven chamber out of firebrick on top..... Overkill or underkill ??? Don't want to do this wrong if I can help it!

Thanks,

Mark