  A naive application of the heavy quark expansion (HQE) yields theory
estimates for the decay rate of neutral $D$ mesons that are four orders of
magnitude below the experimental determination. It is well known that this huge
suppression results from severe GIM cancellations. We find that this mismatch
can be solved by individually choosing the renormalisation scale of the
different internal quark contributions. For $b$ and $c$ hadron lifetimes, as
well as for the decay rate difference of neutral $B$ mesons the effect of our
scale setting procedure lies within the previously quoted theory uncertainties,
while we get enlarged theory uncertainties for the semi-leptonic CP asymmetries
in the $B$ system.
