The ISO index on the camera changes the overall exposure including "re-rating" the film. That control alone won't accomplish what you are trying to do which is down rate the strobe for fill.
However, the SB-20 also has a "A" exposure system that meters the flash exposure from the reflected light from the scene (does not use TTL, but a separate sensor in the strobe unit). I don't understand how it could give an accurate exposure if the strobe-unit itself doesn't know about the ISO setting you've chosen for the film (e.g. what you've set on the camera). I believe you have to match the camera setting to the strobe setting using these control to get an accurate exposure.
Anyone else care to shed some light on this?
I actually think you could test this by setting the strobe to "A", choosing different ISO settings on the strobe and then using the open-flash button to trigger the strobe. I think you should be able to see a visible difference in the strobe duration between the minimum and maximum ISO setting.
Roger It's still, ISO, aperture and shutter-speed, right?