An early measure of good chair posture was to locate a line through the thigh (from the hip to the knee) and the torso (hip to shoulder). Good seated posture was presumed when angle of these lines, or the thigh-torso angle was greater than 90-100 degrees.

This photo shows a thigh-torso angle (shown with the blue lines) greater than 90 degrees, which would seem to indicate reasonable torso and spine posture. 

When we look more closely at the relative position of the pelvis, or the base of the spine and the head, we can see the spine in nearly full forward bending. This spine may not bend much farther!

To measure specific angles between body parts when at work may not reliably show the inside position of the spine. Another problem to measure an angle for the ideal posture, is that the body may change positions as work changes.

The simplest and most sensitive test for good spine posture is to simply check for some slight movement at the low back by extending the torso.  This movement test can easily be done in a chair, and at any time during the work process.