You have 5 stops of adjustment over a six stop range. For example if you are at full power, 500Ws, you can come down five stops to 15Ws. The settings on the flash would be :Stops dial 1, Tenths dial 0.0, for 15Ws, now go up to 5 on the Stops dial and you have gone up four stops, then go to the Tenths dial and go up 0.9 stops for a 4.9 stop adjustment range across a 5.9 stop output range.
If they had a zero on the Stops dial and you could input just 0.1 on the Tenths dial you would have full adjustment range of your actual output range.
You are right, the numbers are straightforward, 400Ws is close to 1/3 stop less than 500Ws and 250Ws is one stop less than 500Ws.
great - thanks for the clarification. i was debating getting a 250W light for the background. I often find that even with my main lights turned up all the way, and the background light turned down all the way - the BG is still too bright. this is because i often have large modifiers on my main lights (which eats up the light), and just a standard reflector on the BG.
a 250W light should essentially be 1/2 the brightness at its lowest stop compared to the 500W at its lowest stop, which would be great. I would also assume that the 250W at full power would be the same thing as my 500W at one stop below full power. clearly, the recycle time on the 250W is going to be slower, but I think it will be ideal.