As far as I know, the guide number is the distance the flash would reach at full power. Unless otherwise stipulated I believe it assumes ISO 100 and f/1. In the case of the built in flash on the D200 that's 12 meters or 39 feet (manual 13 meters and 42 feet).
So if you increase the f-number the distance will shorten. Conversely if you increase the ISO the distance will lengthen.