I feel compelled to post my experience with a higher end telescope coupled to a lower end camera & reducer. All Zwo & QHY cameras have a margin of error in their back focus and guess what? So do most of the available reducers/flatteners. They just don't tell you and neither does ZWO, which is a disservice "correct me if they changed this".. So when using two items that have one half millimeter margin of error, you are looking at a variable of 2mm off from 55mm most need to achieve. Meaning, the required spacing may be as low as 54 or as high as 56.
That may sound trivial, but I never knew how critical this spacing was until I hit it on the nose. Everything just changed... kind of like going from 480i to 480p. If you have ever experienced that, you know exactly what I am talking about. The stars just jumped out and the focus went from an average in SGP of 1.69 to a 1.45 as soon as i hit it. I suspect there are many astrophotographers out there who have no clue to the spacing issue and think they are getting the best out of their setup. Unless you are on the money you are not.
What helped me the most was .15mm washers and a baader varilock. I am going to take the exact measurement of my spacers now and get my first custom spacer from precise parts and never take it off ever again.
Hope this helps other new astrophotographers.