Yes, I was incorrect about the keystone terminology.  However, this is how I've decided to place the rows and columns in the dome here.  As you can see by the inset grout lines, there's now clearly a keystone at the top where it should be.  I did a 4x3x2 running bond.  The same layout is on the outside of the dome.  It's a bit different than the alcove in the picture, which looks like it has only 2 rows of 5x4 columns.  Not sure if this would be architecturally common or not, but I think it looks good.  I've got a lot more detail I want to add.

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