  A closed-form formula is derived, which corrects for the modulation format
dependence of the Gaussian Noise (GN) model in the presence of inter-channel
stimulated Raman scattering (ISRS). The analytical result enables a rapid
estimate of the nonlinear interference (NLI) for arbitrary modulation formats
and avoids the need for complex integral evaluations and split-step
simulations. It is shown that the modulation format dependent NLI can be
approximated by two contributions, one originating from a single span and one
asymptotic contribution for a large number of spans. The asymptotic
contribution is solved in closed-form for an arbitrary link function, making
the result applicable for generic fiber systems using lumped, distributed or
hybrid amplification schemes. The methodology is applied to the ISRS GN model
and a modulation format correction formula in closed-form is derived which
accounts for an arbitrary number of spans, inter-channel stimulated Raman
scattering, arbitrary launch power distributions and wavelength dependent
dispersion and attenuation. The proposed formula is validated by numerical
simulations over the entire C+L band for multiple fiber types.
