You need to check the website where you purchased the field flattener. That is usually where you will find the specs for required backfocus. Typically, however, this is 55mm give or take a little bit (it may require some adjustment to get the corners just right). I don't think any field flattener comes with instructions today. All info is online.
As others have mentioned, you will probably also require another extension tube... probably a 2" tube would do it... in order to get focus.
Have you tried to get focus without the flattener so you have a starting point? That would be the easiest and most logical.
Explore Scientific iEXOS-100 and EXOS-2GT (non-PMC)
Explore Scientific ED 102mm Refractor