I measured and rough-cut the first few clapboards to length, offsetting them and cutting several at a time.
I worked up from the bottom, marking opposite pieces, then placing them back-to-back and cutting both at once.
I then placed the tapered boards back on the full-scale layout and positioned the next pieces for scribing.
I centered the top piece before tracing it, though, so the grain would run parallel to its centerline and perpendicular to the base.
I next duplicated the half circle, placing a couple of fiber-cement cutoffs side by side and tracing around the plywood pattern.