Luxxeon, that's NOT a keystone.  Not in the architectural sense of a dome.  What you have modelled there is simply a header block at the top of a dome arch.  A keystone, in dome architecture, would be a slab of stone which is at the very center of the dome, and holds together the next two segments.  That block you added isn't that.

If you look at your reference (or the inspiration I should say, since you aren't modelling that exact structure here), you can see the bond areas between each stone in the dome.  You modelled them yourself in your FIRST version of this.  Well, at the very top, in the center, where the verticle bond ENDS, that is your keystone.  You did it right in your first version.  You can still do it in this one if you want to, and maybe you were planning on it anyway, but just letting you know, that block wouldn't be a keystone.

Unless you add the running bonds to indicate that there are masonry units involved in creation of this dome, which would meet at the apex, or keystone, then what you have here is not a true arcuate dome.  It's what we call a "false dome".