That is a great observation, and I think you're right in selecting it. I love your design. From a purely mathematical standpoint, there are a couple variables other than the witdh of the base that define the arch. Namely the location of two centers on the x-axis and the third on the y-axis. Not all 3-center arches are close to the d/3.4 ratio. A semicircle is technically a 3-center arch with all three centers on the x-axis (y=0 on the middle center). Likewise, it can also be almost flat if the two centers on the x-axis are at the edge of the arch and the center on the y-axis is pulled way down. The arch approaches flat as the center on the y-axis approaches negative infinity.