Charm Mass Dependence of the $\CO(\alpha_s^2 n_f)$ Correction to   Inclusive $B\rightarrow X_c e\bar\nu_e$ Decay
  We compute the $\alpha_s^2 n_f$ perturbative QCD contribution to semileptonic B decay, including the finite mass of the charm quark. This result provides an estimate of the size of the two-loop correction, which is found to be about 50\% of the one-loop correction. We use these results to set the scale for the one-loop correction using the scheme of Brodsky, Lepage and Mackenzie and find a BLM scale of $\mu_{\rm BLM} = 0.13\, m_b$, when the inclusive semileptonic rate is expressed in terms of the $b$ and $c$ quark pole masses and the \msbar strong coupling. The two loop correction lies roughly midway between that obtained at $m_c=0$ and that obtained in the Shifman-Voloshin limit $m_b, m_c\gg m_b-m_c\gg\Lambda_{\rm QCD}$ while the corresponding BLM scale is somewhat closer to that obtained in the former case. 
Michael Luke, Martin J. Savage and Mark B. Wise