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The CFZ is the focus zone where there is no more improvement in the scopes resolution.
The diffraction limited resolution is defined by aperture size, Starizona use the formula of 140arcsec / aperture in mm (though I've seen other websites use 120/aperture).
So 100mm is 1.4arcsec, 200mm is 0.7 and a 400mm scope is 0.35 arcsec resolution.
As CFZ is defined by Focal ratio, and if each of these scopes were F5 then the CFZ for each is approx 66microns.
If the seeing limits our resolution to 2arcsecs, then we just multiply the 66 microns by the difference in resolution.
So the 100mm scope is 66micons x 2arcsec/1.4arcsec = 94micron CFZ
The 200mm scope 66 x 2/0.7 = 188 microns, and the 400mm is 376 microns CFZ.
The 2 arcsec seeing blur is an average over time, so there will be brief moments when we do get diffraction limited resolution.
All this is academic and doesn't really solve your problem off course, but I thought I would explain my understanding of whats going on.
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