I actually wouldn't count on the tripod ring area being the
no-parallax 
point of the lens. It's never that simple, damnit.
The easiest way to find the no-parallax point of the lens is to pray that it's in the panotools wiki's "
Entrance Pupil Database".
If it isn't, though, then you have to
do tests.
The easiest way is to set up two vertical markers of some kind that line up (say, a lamp post and a door edge, one behind the other from a given viewpoint. Then, rotate the lens a little around what you
think is the no-parallax point, and if the two markers
stay lined up, then you've found it. If they don't, then you need to adjust the rotational point of the lens and try again.
An adjustable panohead (not to be confused with a panning head) is indispensible for this kind of thing. I highly recommend the
NodalNinja 3, but I've only used it with a fisheye lens, not a telephoto. You might have to get the 5 [longer rails] for a larger camera/lens set up.
Now, may I ask why you need to find the no-parallax point of a larger telephoto lens? You're unlikely to be dealing with a lot of parallax error in panos if you're shooting, say, high-resolution landscapes with everything far away from you. It's only if you want to do the gigapixel thing
indoors or with objects in the near foreground that you need to be that precise. For the outdoors stuff you could probably even get away with handheld if you wanted.