Hi Peter
The main thing is that the distance from both chips are exactly equal so both come to focus at the same point. Basically you need to setup the OAG so it fits then measure the distance from the face of the prism to the face of your imaging chip (find out the backfocus for your camera). This is the distance you must set your guide camera chip to. I have to add about 60mm of T2 extensions to the guide cam side to get close ( QHY9 + FW + QHY5). I then use the thumbscrew on the side of the prism stalk to get the exact focal point. Be careful when rotating the camera (hang on to it) as the three tags that lock the lot together have limited purchase and will come free as soon as you back off the thumbscrews which hold the main body to the adaptor.
Good luck
Mark