Using the Screw modifier, how do I accurately set the axis of rotation? I got two good answers to my previous question about using a profile to create a mesh.  The problem I am still having is that I can't get the axis of rotation in the right place.  One thread said to position the 3D Cursor on the axis and another said to move the axis to the Origin.  I redid this three times. The last time was the closest. Still, the axis of rotation varied from ideal. How do I position the 3D cursor more accurately so as to make the axis of rotation where I want it to be?
The modifier uses the objects (local) axis (X, Y, Z)

Modeling from top side or front ortho view will make things simple. use numpad5 to switch orthogonal/perspective view

Shape modeled in top ortho view (green line is Y global axis)



Screw modifier set on Z object axis



Screw modifier set on Y object axis



Once modeled, you can move your object or rotate it (object mode) but in edit mode, you'll get a mess. Apply modifier or clear rotations for editing.