[glend (Glen)]

You need to remember "The Rule of Thirds" is relation to focal length changes as a result of adding glass in front of your camera sensor. If you have filters in the path they will effect the focal length. How thick are your filters? Is there a glass cover over the camera sensor compartment? Forget the cover glass on the sensor itself, but many cameras have sensor comparnents where they may have gas filling, dessicant, etc. That glass counts.
For example, say your filters are each 2mm thick, and you have 1mm cover glass on the camera compartment, that totals 3mm, under The Rule of Thirds, you will need to add 1mm of spacing.
Secondly, most Flatteners have a range in which they can work, try it and see.

My personal preference has always been to use a guidescope, it just makes life easier; unless your dealing with long focal lengths and you can pick a star with the prism. If your starting out, stick with guide scopes imho.
What sort of imaging scope is this you are using?