Interesting discussion. I'm assuming there must be some wavelength factor in the FWHM...although I'd expect tighter stars from shorter wavelengths...
The other night from our little back yard in Brissie I was taking some Ha and OIII subs of the Tarantula. Seeing must have been decent, as happens occasionally. Plugging them into above formula suggests I should be seeing 2.7 arc second stars. PixInsight's subframe selector measured my best of the night at 1.9 and the mean of 2.3 over ~100 subs.
