Wow, I just noticed something. n-sided brush and cylinder are not in the make list. That's not good.
If you use cylinder, you'll have a lot more control. Then you can just use a clipping plane to slice it in half. From there you just drag the two brushes on the end down to the desired height The problem with arch is that it uses a %, not an exact number of units (though you could probably calculate it), so if you try to ever build anything around that arch, it could get ugly and lead to a lot of brush overlapping.
