Hearth slab: castable refractory cement?; DIY - coverage

Hello all, just pricing out a possible brick oven project.  I'm weighing how much more costly it would be to go with castables v. my terrible back, and PITA to do my own mixes.

Having some trouble finding castable refractory concrete.  Plenty of cement, mortar.  I do see Heatcast 40, but I can't tell if that's a concrete ready to pour for a slab, or I need to add in components like below.  Also, hoping to come in much less costly (just a hearth slab using this alone comes to $1800!).  Any recommendations for the pre-mixed refractory concrete?

If I do go the DIY route, I was using the recipe for refractory concrete of:

Mixture: (parts ratio is 3 x 2 x 2 x 0.5, plus water)
3 shovels of the gravel or grog/crushed firebricks
2 shovels of sand (Portland concrete type only)
2 shovels of the cement
1/2 shovel of lime (for Portland concrete type)
This amount will require approximately 6-7 liters of water to mix the concrete.

My slab is 8.7 c.f.  How do I go about calculating the volume of the above recipe?  Meaning, I can buy the cement with known coverage, but obviously once I add in the other things I will have much more than needed for the slab (I think.  I'm a space cadet).  Anyone have any guidance on figuring out how much cement (and therefore, all the rest of the components) to buy?

Thanks, all.