June 18th, I am back to building after a brief work related stop. After letting the hearth base cure for a few weeks, the shims are removed, and I am pleased when the entire thing does not collapse. That's quality engineering!!! 
Notice that the entire hearth is supported by rebar - this is the way it is supposed to be built. 

I am just doing a mockup of the hearth bricks here, simply stacking them up to visualize the next step. No mistakes here. 

Everything has to be perfect here. The oven opening has to be at exact placement, height and width to not only be architecturally sound and usable, but also to provide the most efficient characteristics of heat retention of the oven.
The math: the oven opening height has to be at a ratio of 62-64% of the height from the top of the inside of the oven dome.

After I did the mockup, all of bricks are removed, and re-set into a bed of fireclay/sand mix. The fireclay/sand mix is essential, and we cannot use concrete at this point because the concrete will just burn away with the high heat. 
The walls go up shortly afterwards, set in a fireclay cement which is 10 parts sand, 6 parts fireclay, 3 parts lime and 2 parts portland cement. It is a breeze to work with, spreads easily but dries extremely fast - each fire brick has to be soaked in water for a few minutes before laying it, or the brick instantly absorbs the moisture and the cement mixture "sets". That's bad, because like all cement, it needs time to cure, and drying it will make it weak.