  We have used the ratio of column densities (CDR) derived independently from
the 850-$\mu$m continuum JCMT Plane Survey (JPS) and the $^{13}$CO/C$^{18}$O
$(J=3-2)$ Heterodyne Inner Milky Way Plane Survey (CHIMPS) to produce maps of
the dense-gas mass fraction (DGMF) in two slices of the Galactic Plane centred
at $\ell$=30$^{\circ}$ and $\ell$=40$^{\circ}$. The observed DGMF is a metric
for the instantaneous clump-formation efficiency (CFE) in the molecular gas. We
split the two fields into velocity components corresponding to the spiral arms
that cross them, and a two-dimensional power-spectrum analysis of the spiral
arm DGMF maps reveals a break in slope at the approximate size scale of
molecular clouds. We interpret this as the characteristic scale of the
amplitude of variations in the CFE and a constraint on the dominant mechanism
regulating the CFE and, hence, the star-formation efficiency in CO-traced
clouds.
