  We analyze the perturbative and parametric stability of the QCD predictions
for the Callan-Gross ratio $R(x,Q^2)=F_L/F_T$ in heavy-quark leptoproduction.
We consider the radiative corrections to the dominant photon-gluon fusion
mechanism. In various kinematic regions, the following contributions are
investigated: exact NLO results at low and moderate $Q^2\lesssim m^2$,
asymptotic NLO predictions at high $Q^2\gg m^2$, and both NLO and NNLO
soft-gluon (or threshold) corrections at large Bjorken $x$. Our analysis shows
that large radiative corrections to the structure functions $F_T(x,Q^2)$ and
$F_L(x,Q^2)$ cancel each other in their ratio $R(x,Q^2)$ with good accuracy. As
a result, the NLO contributions to the Callan-Gross ratio are less than 10% in
a wide region of the variables $x$ and $Q^2$. We provide compact LO predictions
for $R(x,Q^2)$ in the case of low $x\ll 1$. A simple formula connecting the
high-energy behavior of the Callan-Gross ratio and low-$x$ asymptotics of the
gluon density is derived. It is shown that the obtained hadron-level
predictions for $R(x\to 0,Q^2)$ are stable under the DGLAP evolution of the
gluon distribution function. Our analytic results simplify the extraction of
the structure functions $F_2^c(x,Q^2)$ and $F_2^b(x,Q^2)$ from measurements of
the corresponding reduced cross sections, in particular at DESY HERA.
