May I wish you a warm welcome to the CameraLabs forums.
I think the term you need to use is "optical center", sometimes referred to as "nodal point", rather than "optical axis". As I'm sure you know, positioning the camera so that the panorama head's rotation axis passes through the optical center requires a bracket such as
If you are lucky then Tamron's technical support guys might be able to provide you with the exact distance along the barrel of your lens' optical center. Alternatively, there is a tutorial here
at the Really Right Stuff site and they also provide a link to another site
from which you can download this PDF document
with a much more detailed guide.
Discovering the optical center will take some time but once it's done and the camera bracket is marked appropriately it should never need repeating so I guess it's worth some effort to get it right.