Alright, so I am creating parts and drawings from exisiting drawings that came from AutoCAD, so the customer can see the parts in 3D.  However, I am having a little trouble with the way they dimensioned some of the parts.  The parts are mostly rings or cylinders, and the drawings all have section views to show the cross-section of the rings.  I want to dimension the diameter of the rings to the line in the section view (see attached image), but I can't figure it out.  I have tried making the dimension in the front view, and then holding shift and dragging it over, but all I get is a red circle with a cross through it.  Are there any tricks to get this to work?  Any help is appreciated...  Thanks.