  By combining an adiabatic approach based on a `locally monochromatic'
approximation with a local Hilbert transform, it is demonstrated how vacuum
birefringence in the strong field regime can be calculated using a rate
approach suitable for Monte Carlo simulation codes. Results for the flipping of
the photon's polarisation (helicity) are benchmarked with evaluation of exact
expressions in a circularly (linearly) polarised plane wave of finite extent.
Example probabilities are given for typical experimental parameters.
