My experience with flatteners is that the shorter the scope FL, the longer the spacing between flattener and camera. I use an AT2FF with a 600mm Vixen ED80 and I get the best field correction at about 65 mm of spacing. Just throwing that out there as an idea on what kind of range you can try. The best spacing may vary a lot from one FL scope to another of greatly different FL.
Thanks Adam. I'll try some longer spacing too.
But to be clear, I bought this scope primarily as a grab and go visual scope. Imaging was only a secondary purpose. I figured it would make a fun to use wide field imager based on my previous ATED72 original model and I think it will perform that role nicely...John