  We show that the effective decay asymmetry for resonant Leptogenesis in the
strong washout regime with two sterile neutrinos and a single active flavour
can in wide regions of parameter space be approximated by its late-time limit
$\varepsilon=X\sin(2\varphi)/(X^2+\sin^2\varphi)$, where
$X=8\pi\Delta/(|Y_1|^2+|Y_2|^2)$, $\Delta=4(M_1-M_2)/(M_1+M_2)$,
$\varphi=\arg(Y_2/Y_1)$, and $M_{1,2}$, $Y_{1,2}$ are the masses and Yukawa
couplings of the sterile neutrinos. This approximation in particular extends to
parametric regions where $|Y_{1,2}|^2\gg \Delta$, {\it i.e.} where the width
dominates the mass splitting. We generalise the formula for the effective decay
asymmetry to the case of several flavours of active leptons and demonstrate how
this quantity can be used to calculate the lepton asymmetry for
phenomenological scenarios that are in agreement with the observed neutrino
oscillations. We establish analytic criteria for the validity of the late-time
approximation for the decay asymmetry and compare these with numerical results
that are obtained by solving for the mixing and the oscillations of the sterile
neutrinos. For phenomenologically viable models with two sterile neutrinos, we
find that the flavoured effective late-time decay asymmetry can be applied
throughout parameter space.
