OAG Spacing and Setup

[nightster]

I am researching adding an OAG for various reasons (1. possible flexure in my Side by Side setup; 2. prep for a longer FL scope this fall; 3. glutton for punishment) and looking at which extensions and spacers necessary to come to focus. Behind my Stellarvue SFF6 flattener I need the standard 55mm. I have the SX USB FW (-29mm) and then the 460ex (-13mm.) Also accounted for are my 1mm Astronomik (+.3) filters. I currently use 13mm of spacers to fill the gap. I believe the SX OAG is also 13mm so I can pull the spacers and should be good. What I can't find data on is the backfocus of the optical path of the OAG itself. Not how affects the 460 but how if affects the Lodestar. Will I need the extension between the C threads on the OAG and the lodestar w/ its backfocus of 12.5mm? In other words does anyone know the distance between the prism on the OAG and the C threads?

Also I plan to extend the 48mm bore of the Flattener back to the OAG, removing the OEM 48mm to T thread extension supplied by SVue. I use the 1.25 filters. The thought being there may be less chance of prism shadowing if the mirror is well above the plane of the filters.   Many have told me to stick with a dedicated guidescope, even moving from my solid mounted ST80 to a 50/60mm mini to help my chances of getting 20-30 minute subs. Don't want to spend 70% the cost of an OAG and rely on those 3 screw mini guidescope mounts as they look suspiciously flexy at best. 

What other issues should I be researching if I move to an OAG?