Eric, most everyone (including me) used a 12" height for the inner arch. Mine came to 12.125" in the center. At the outer edges, it depends: I went 3 brick high (on the arch walls) and a small mortar joint so by edges are 8" and peak up to the 12.125". It's got a nice curve. Many others went 4 brick high on the side walls so they went about 10.5" to the 12" center. Depends on if you want a gentle arch or "arch-ey" arch.
If you want to get technical: 63% of a 19.5" center dome height is 12.28". If you make your dome center a full 20" tall, then you can have a 13" door height but (personally) I think shooting for an 'anything just under 20" dome height' is a worthy goal. Too tall and you waste your heat up there. True Naples style ovens have a low dome for pizza's , Tuscan style ovens have a higher dome.
So, dimension 'A' at 20" is perfect. The B & C taper can be what's comfortable for you and what looks good. I think any taper (widening or flared entry) is nice. At 1st glance, it adds a bit of complexity (teensy) but I was able to do it successfully (after copying what Christo did

This pdf is my as-built of all those dimensions you asked about. Please ask again for any other dimension you need or we missed.
-Dino
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