  We point out that, contrary to some recent claims, there is no intrinsic
long-distance uncertainty in perturbative calculation of the QCD effects in the
$t \tb$ and $t \bb$ loops giving the electroweak corrections proportional to
$m_t^2$. If these corrections are expressed in terms of the ``on-shell" mass
$m_t$, the only ambiguity arising is that associated with the definition of the
``on-shell" mass of a quark. The latter is entirely eliminated if the result is
expressed in terms of $m_t$ defined at short distances. Applying the
Brodsky-Lepage-Mackenzie criterion for determining the natural scale for
normalization of $\as$, we find that using the ``on-shell" mass makes this
scale numerically small in units of $m_t$. Specifically, we find that by this
criterion the first QCD correction to the $O(m_t^2)$ terms is determined by
$\as^\msb(0.15 m_t)$. Naturally, a full calculation of three-loop graphs is
needed to completely quantify the scale.
