  Superheavy nuclei produced until now are decaying mainly by $\alpha$ emission
and spontaneous fission. Calculated $\alpha$ decay half-lives are in agreement
with experimental data within one order of magnitude. The discrepancy between
theory and experiment can be as high as ten orders of magnitude for spontaneous
fission. We analyze a way to improve the accuracy by using the action integral
based on cranking inertia and a potential barrier computed by the
macroscopic-microscopic method with a two-center shell model. Illustrations are
given for $^{282}$Cn which has a measured fission half-life.
