How to translate face tracking offset values into a 3D null position I want to put a 2D hat on a video clip of a person, whose head is tilting as he speaks.

I can face-track the face using After Effects CC 2017.
I end up with detailed tracking data, which includes face orientation information.
I can link the orientation information to a 3D null, but how do I link the positional information of the head?
The head appears to only be expressed as a percentage (of what?).  

Is there an easy way to connect this percentage data to 3D positional data on a null?

Equally, if I do a simple face track, I get an animated mask.  What's the right way to translate one of the points from that mask into a null that could be parented to another object?
You can access the position of a point in a mask with expressions (in AE CC 2017+) with expressions. To access the first point of a mask called "Mask 1" on layer called "my Layer" you'd use

pts = thisComp.layer("my Layer").mask("Mask 1").maskPath.points();
pts[0]


note that the points are numbered from 0, so the second point is pts[1], etc.

I'm not 100% sure I understand what you';re trying to do, have you tried doing a standard motion track using the inbuilt motion tracker or Mocha?