If you're asking what the property means, then I gather this from the docs:
"The distance between the eye or the viewpoint's origin (0,0,0) and the display object located in the z-axis. During the perspective transformation, the focalLength is calculated dynamically using the angle of the field of view and the stage's aspect ratio (stage width divided by stage height)."
Or were you asking what my actual value was? Hah, I guess that makes more sense
It was around 604, which doesn't match up with either the 920 I'm seeing for when objects disappear NOR with the expected value according to my trig.