target scale value for scaling every fitted 3DMM

[haijing1995]

In paper,
For every participant, we determined a person-specific inter-ocular distance value by exploiting our knowledge of relative camera positions. This inter-ocular distance (defined as the Euclidean distance in millimeters between the outer eye corner landmarks) is then used as a target scale value for scaling every fitted 3DMM.
Can you explain the process of determining of the target scale?
Thanks