The 55mm flange to film/sensor distance is an oldie, going back to film camera days, where the camera mount T adapter not only matched the 42mm thread to the bayonet/screws/etc of the body, it also varied the adapter thickness to ensure that the T lens flange ended up (hopefully) exactly 55mm from the film plane. That's presumably why the 55mm keeps popping up in spacing specs. If that helps figure out what you need, if post #5 doesn't.
ZWO should have a spec for the camera body flange to sensor distance.
Also, if you want to make small spacing changes, Baader has sets of delrin T spacers at reasonable cost, I am sure others do as well. There is also the Svbony adjustable T thread (42mmx.75mm) spacer. There was also a post on spacers this week with a link to the Svbony.
it's for EAA, and they can end up with unexpected spacer lengths.
One of the Baader spacers might help add or make the the missing .3mm. Amazon and the current Baader importer, AgenaAstro.
The set's thinnest looks like .6.
Edited by markb, 15 July 2020 - 04:38 PM.