has anyone tried to translate the numeric zoom values into estimations of focal length? Obviously zero is 5.4 and 100 is 108.

- but does that mean 50 on the scale is approx 55mm? Is the progression linear?

There are some answers if you search the forum.

http://www.dvinfo.net/conf/canon-xh-series-hdv-camcorders/81683-xh-a1-zoom-number-vs-35mm-equivalent-focal-length-2.html

For what it's worth, the progression is curvilinear (not rectilinear).

many thanks - EXIF it is