Focus distance is measured to the focal plane mark, a circle with a line through it, on body, near rear edge of the top LCD. This marks the location of the digital sensor or film.

I have to say, "forget it". It could work, but only at one zoom value. Long story, but you will have extreme problems marking it, because zooming changes things considerably. On lenses even with a distance scale, the same focused distance (on very many lenses) will most likely be reported as a different lens rotation with rather different numbers at each zoom (different for the same focused distance). So you would need a new scale for every zoom value.

The reported focused distance is already in the Exif data (manufacturers section, but you may need a pretty good and recent Exif viewer to see it - ExifTool.exe is a good one). And that Exif focused distance is very often blatantly wrong, because many lenses usually report different distances with different zooms at the same distance.

My own complaint about it (a pet peeve) is how the Nikon D-lens distance is inexcusably wrong, sometimes terribly wrong, but is still allowed to seriously affect default flash mode exposure of TTL BL direct flash. We need a new menu option to refuse to allow that.

http://www.scantips.com/lights/ttlbl-d.html (half way down page)

Third party flashes seem able to sidestep the TTL BL error (no head tilt switch)

http://www.scantips.com/lights/ttlbl-cords.html

but most Nikon flashes suffer from it. It seems unimaginable. But only TTL BL direct flash is affected... which is default mode, but not bounce, not non-balanced TTL flash mode, etc.