The second number is the GN in feet, 28 meters equals 91.9 feet.
Take your GN in feet and divide it by your subject distance in feet to get your f stop. So if your subject is 10 feet away and you have the flash set to 24mm then you get an f stop of 91.9 feet/10 feet = f9.2. If your flash is set to 200mm then 196.9/10 = f19.7
OK, I get it thanks, so you're just plugging in F = GN/d. So with GN = 28 and d = 10m, then F = 28/10 = 2.8. Cool.
It's not immediately clear in the manual's GN chart though that "flash coverage" actually equates to "flash zoom". Can we actually assume that the flash zoom head is set to the focal length (i.e. the horizontal row labeled "Flash Coverage" in the chart)?