  The correct stacking of hexagonal layers is used to obtain accurate estimates
for the exchange and anisotropy parameters of the geometrically-frustrated
antiferromagnet CuFeO$_2$. Those parameters are highly constrained by the
stability of a collinear metamagnetic phase between fields of 13.5 and 20 T.
Constrained fits of the spin-wave frequencies of the collinear $\uudd $ phase
below 7 T are used to identify the magnetic unit cell of the metamagnetic
$\uuudd $ phase, which contains two hexagonal layers and 10 Fe$^{3+}$ spins.
