Need help with oven size relative to hearth and placement etc

[Yokosuka dweller]

Dear Community,

My hearth will be as follows in the attached diagram.

I think I'd like 80cm diameter oven. Is this good size compared to the overall size of my stand? Should I place the diameter exactly in the center of the slab, or how much extra space should I calculate as necessary for the oven entrance, insulation on the sides etc. Could anyone with more experience and expertise please share some tips so I get it right.

Thanks!