  We calculate the part of the order $\alpha_s^2$ correction to the
semileptonic heavy quark decay rate proportional to the number of light quark
flavors, and use our result to set the scale for evaluating the strong coupling
in the order $\alpha_s$ term according to the scheme of Brodsky, Lepage and
Mackenzie. Expressing the decay rate in terms of the heavy quark pole mass
$m_Q$, we find the scale for the $\overline{MS}$ strong coupling to be $0.07\,
m_Q$. If the decay rate is expressed in terms of the $\overline{MS}$ heavy
quark mass $\overline m_Q(m_Q)$ then the scale is $0.12\, m_Q$. We use these
results along with the existing calculations for hadronic $\tau$ decay to
calculate the BLM scale for the nonleptonic decay width and the semileptonic
branching ratio. The implications for the value of $|V_{bc}|$ extracted from
the inclusive semileptonic $B$ meson decay rate are discussed.
