I find the peak pixel in the measured PSF.  Then measure the enclosed power
in a circle with variable radius centered on the peak pixel.  I normally use
2.5"  I compute the diffraction pattern for a centrally obscured disk at one
wavelength, usually the central wavelength of the filter.  This diffraction
pattern is computed at much higher sampling than the PSF.  I then bin the
diffraction pattern to the pixel scale of the measured PSF.  I then weight
the binned diffraction pattern by the encircled power of the PSF. I then
ratio the maximum of of the PSF to the maximum of the diffraction pattern.