I don't have a Strehl measuring package per se but use some existing programs of mine.

I firstly generate peak-centered images of the measured data by performing a Fourier shift
on the data.  The peak location is found to the nearest pixel and then a 3 x 3 pixel box
(for Nyquist sampling) oe a 5 x 5 box (for the oversampled data) is used to compute the
sub-pixel position of the peak. The image is then Fourier shifted, i.e. by adding a
tilt-term to the Fourier phases to have the peak centered on a single pixel. This is an
iterative process and permits me to shift the image to within 1/5 pixel.

If I don't have the pupil info, then I generate a weighted pixelated pupil at a spatial
scale comparable to the image scale of the data, i.e. the radius of the cut-off frequency
of the MTF is the diameter of the outer pupil. I then numerically create a PSF by taking the
power spectrum of this pupil using an FFT. Initially I used to make sure that the array side
was the same as the data so that the PSF was generated with the correct sampling. This has since
been modified, by simply zero-padding in the Fourier domain, to generate an oversampled PSF,
typically 4x Nyquist, which is then "binned" using the iraf "blkavg" task. If the peak of this
PSF is not on a single pixel, then I perform the same Fourier shifting as above.

For the Nyquist sampled data I normalised the volumes in a 256 x 256 pixel square.