  The QCD corrections for the box diagrams are revisited for the case of a
heavy top quark with $\mt = 174 \GeV$. We resolve first a longstanding
discrepancy between two methods of calculation by showing that they give the
same results when the threshold factors are treated correctly. Using this
observation we refine our earlier results and derive formulae valid for the
$K$- and $B$-meson systems. Our formulae are given in terms of integrals to be
evaluated numerically, as well as approximate analytical formulae. These
calculations include the evolution above $\MW$ which has been neglected by
other authors.
