  We discuss in depth the validity and limitations of a theoretical scheme to
evaluate photo-electron spectra (PES) through collecting the phase oscillations
at a given measuring point. Problems appear if the laser pulse is still active
when the first bunches of outgoing flow reach the measuring point. This limits
the simple scheme for evaluation of PES to low and moderate laser intensities.
Using a model of free particle plus dipole field, we develop a generalized
scheme which is shown to considerably improve the results for high intensities.
