The model of the field flattener is crucial in determining the correct back focus for best results. Look it up on WO’s website as they usually list the specs for discontinued products as well as their current offerings.
55mm isn’t so much a “standard” as an adoption encouragement, based on the back focus requirements of a Canon DSLR plus a T-adapter. This is purely a “convenience”, depending on your point of view (or an inconvenience for those of us who dare using something more). Many flatteners provide more than this - including most of WOs recent flatteners.
Anyways, from your image of M20, you need to add more space in front of your main imaging camera. Consequently, you’ll need to add more to your OAG too…the same amount, assuming they are currently both in focus in your rig pic. I’d suggest putting this scope side of the OAG, as you’ll then only need to use one adapter instead of two
