Uses Machin Eq 12.10 (p. 199), i.e. the design effect plus the Student t
correction term z^2/2, and rounds ONCE at the cluster level. Anchor:
Machin Example 12.1 (p. 206) - delta=3.5, sigma=6.94, m=2, ICC=0.025,
2-sided alpha=0.05, power=0.90 -> N=171.35 -> 172, K=86 (43 per arm).