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].