with the same camera, but with Celestron C9.25 with f/6.3 reducer (= 1457 mm effective focal length), on a good night I usually get HFR values around 2 pixels, from a non-saturated, medium-bright star. Depends on star brightness, exposure time during AF and very much on seeing conditions.
Translated into arcseconds: alpha = arctan(object size / focal length)
In my case object size is 18.5 microns (2 pixels radius = 4 pixels diameter = 4 x 4.63 microns), so my half-flux discs have a diameter of 2.6“.
In your case (100ED has FL = 550 mm, right?): Object size = 13 microns (2 x 1.4 x 4.63), so your half-flux star discs are 4.9“.
Normally, resolution is seeing-limited, 1“ would be excellent (mountains, no turbulences, dry air) and 4“ would be bad seeing. I would expect your discs can go down to 3“ or 2.5“ as well resulting in a HFR size of 1 pixel or slightly below that.