I don’t have any handy right now, but I don’t think that piece will generate much of a point at the top of your window, and possibly not one at all. You’d probably be better off simulating it using a combination of 75- and 45-degree inverted slopes. You could also try the 1x3x3 arch capped with 45-degree slopes but without trying it, I’m not sure if that will look okay or not.
I can’t think of any pieces that are the right size for that dome, you might wanna get Arthur Guigick’s free dome software and design it that way.