How to translate face tracking offset values into a 3D null position
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?