Most lens will have mark for the rear nodal point. Its the rear nodal that you want to be your rotation point. James have you ever used a nodal ninja for this type of work? It can be helpful as its designed to do this exactly thing. I don't use one so I don't know if they really work well but I know friends that use them.
What it can't do is work like a bellows with rise/fall/tilt/shift which will allow for changing the plane of focus which is very important at such high magnifications.
You can probably build a blows pretty cheap and easy if you are willing to give up auto settings.
