Here's a word document on how to dimension using a named centerline.
This is for NX8.5 and earlier. From NX9 and beyond it's a totally
different process using a "name selection" box and a few other different things of course...
Or you can just create and name the centerline in modeling under properties
and then you can reference it in drafting as your first pick without
going in and out of view dependent edit.
