What units do you use? centimeters or inches. And what do you use for the light emission, which position? On the webcam?
I believe the values are only compared against each other, so units of measurement are removed from consideration.
I use a webcam with built-in IR LEDs (marketed for low-light use).
An additional tweak I've since made to the TrackIR reflector clip was to take some opaque tape and mask much of the reflector surface. This results in smaller points for Free-Track to follow and gives smoother tracking.
After doing this, I further "refined" the values I use for Free-Track's model dimension. Your milage may vary here, since these values now are the distances between points exposed on the reflector surfaces by selective masking--but the points are centered on each reflector and other clips' reflectors' centers should be quite similar.
Now I use:
In the Free-Track GUI, from left to right-- [188] [400] [300]