  Using 5 GHz radio luminosity at light-curve maximum as a proxy for jet power
and black-hole spin measurements obtained via the continuum-fitting method,
Narayan & McClintock (2012) presented the first direct evidence for a
relationship between jet power and black hole spin for four transient
black-hole binaries. We test and confirm their empirical relationship using a
fifth source, H1743-322, whose spin was recently measured. We show that this
relationship is consistent with Fe-line spin measurements provided that the
black hole spin axis is assumed to be aligned with the binary angular momentum
axis. We also show that, during a major outburst of a black hole transient, the
system reasonably approximates an X-ray standard candle. We further show, using
the standard synchrotron bubble model, that the radio luminosity at light-curve
maximum is a good proxy for jet kinetic energy. Thus, the observed tight
correlation between radio power and black hole spin indicates a strong
underlying link between mechanical jet power and black hole spin. Using the
fitted correlation between radio power and spin for the above five calibration
sources, we predict the spins of six other black holes in X-ray/radio transient
systems with low-mass companions. Remarkably, these predicted spins are all
relatively low, especially when compared to the high measured spins of black
holes in persistent, wind-fed systems with massive companions.
