  We present a fitting function to describe the statistics of flux modulations
caused by interstellar scintillation. The function models a very general
quantity: the cross-correlation of the flux observed from a compact radio
source of finite angular size observed at two frequencies and at two positions
or times. The formula will be useful for fitting data from sources such as
intra-day variables and gamma-ray burst afterglows. These sources are often
observed at relatively high frequencies (several gigahertz) where interstellar
scattering is neither very strong nor very weak, so that asymptotic formulae
are inapplicable.
