General Rule of Thumb for DOF?

20droger wrote in post #6065767
You NEVER have a zero DOF.

Although sometimes it feels like that when shooting macro

how do distance to subject and focal length relate, if at all?
Eg given the same framing/subject size in viewfinder, what would give a shallower dof - a wide lens close in, or a long lens further away?

Depends. You really need to play with the DoF calculator to learn the interrelationships between the variables:

The bigger the aperture, the shallower the DoF.
The longer the focal length, the shallower the DoF.
The closer the subject, the shallower the DoF.
The bigger the sensor, the shallower the DoF.

The latter is the least important, unless you're using multiple cameras.