Dude, I don't know but I eagerly wait for the answer.... Wouldn't 1/8, even at dusk still make your main subject (that you are flashing) kind of blurry too?

I've experiemented with metering the background with 1/30, setting the camera on manual to 1/30 at, say, F4 and flashed the main subject with my vivitar 2 stops lower... The jury is still out on the results but it looked a lot better than my past, full flash crap...