According to the description in helpfile, the distortion focal length is calculated by tracing real rays but with small fields definition, for both situations. 

The distortion focal length can be decided by Yref and tanθ（take F-Tan(theta)）as example. 
Yref:  The reference ray height is computed by tracing a real ray from a very small field height, and then scaling the results as required. Using operand RAGY can extract the height value at image surface.
tanθ: use (REAB/REAC) to calculate this value. Note that θ is the angle in object space.
 
Get distortion focal length according to the equation: f= Yref/tanθ

Take \Zemax\Samples\Sequential\Objectives\Wide angle 100 degree field.zmx as example. 

And another example system with finite object distance: 
\Zemax\Samples\Sequential\Objectives\Double Gauss 28 degree field.zmx Object distance 400.