How do I radial dimension in this case?

Radial dimensions are measured as though the object is projected to the construction plane. Can I do a radial dimension of the object projected to the plane that the curve actually lies in? I imagine so but I don’t know which of the many things that I don’t know how to do yet will enable this. Hints appreciated.

Dimensions ALWAYS are created coplanar to the current CPlane.

If I understand your description correctly, you will need to change the viewport CPlane (probably use the 3-point option), to make a CPlane that is coplanar to your curve.

Then do your dimension and switch back to the expected CPlane.
Consider naming the special CPlane so you can get back to it easily if needed.

Thank you. It appears to be time I learn how to manipulate CPlanes.

Handy tools when you need them.
SetCPlane in the viewport drop-down menu.