Also see this from Texas Nautical Repair (Takahashi America). They're very knowledgeable if you have queries. Here are links to measurements on my setup (TSA-120 with 2.7" focuser). The camera is probably different but there will be some info there that applies to your setup.
The reducer comes with a couple of rings that can be inserted to progressively reduce the image scale, so the distance for this isn't so critical.