I jury rigged an extended lensboard to my 8x20 Canham so I could verify focus and bellows draw...

At infinity, bellows draw is 39.4" (1000mm), the same as the focal length.
Cranking the lens out to about 43" (1090mm) brings focus to 200' (60 meters).
To focus at 100' (30 meters), bellows draw would have to be about 46" (1170mm).
An extended lensboard could be quite handy...

All tests were wide open, I didn't attempt to evaluate depth of field.

Reinhold