  In a somewhat forgotten paper [1] it was shown how to perform interpolations
between relativistic and static computations in order to obtain results for
heavy-light observables for masses from, say, $m_{\rm charm}$ to $m_{\rm
bottom}$. All quantities are first continuum extrapolated and then interpolated
in $1/m_h=1/m_{\rm heavy}$. Large volume computations are combined with finite
volume ones where a relativistic bottom quark is accessible with small $am_{\rm
bottom}$. We discuss how this strategy is extended to semi-leptonic form
factors and other quantities of phenomenological interest. The essential point
is to form quantities where the limit $m_h\to\infty$ is approached with power
corrections O$(1/m_h)$ only. Perturbative corrections
$\sim\alpha_s(m_h)^{\gamma+n}$ are cancelled in the construction of the
observables. We also point out how such an approach can help to control
systematics in semi-leptonic decays with just large volume data. First
numerical results with $N_f = 2 + 1$ and lattice spacings down to 0.039 fm are
presented in [2].
