Here are my results and methods:

For this simple case, I assumed the images
were completely background subtracted, flat-fielded etc
so I skipped those portions of my code. I directly
applied Don Gavel's IDL strehl funtion with parameters
        lambda = 1.635 microns
        pixel size (arcsec) = .0165966 [s4r case], .0331932 [s2r case]
        D (m) = 5.08
        D secondary (m) = 1.8288

I used as wide a FoV as possible. Restricting it led to slightly higher
strehls.

Notes on Don Gavel's algorithm.  He used a 2 lambda/D box around the peak of the PSF to find the sub-pixel centroid.  He then generates a perfect PSF numerically from the pixelated pupil with the shift (applied via Fourier phases) so the peaks are co-located. He then measures the relative strengths of the two peaks.  I am not sure what sized area is used for the normalization. Note that no binning is performed on the PSF which will account for the underestimate of the Nyquist strehls. - JC