I don't use specific CHDK settings.
My external flash has a chart (on the back) that vary upon ISO, TTL, short distance auto, long distance auto and manual, with 1/4, 1/2, 3/4 and full power. With these information, I set the "f" at 1/30 sec. That's it!
The light sensor shall "see" my flash, period. I can use full internal flash power without problem, my external flash is a lot more powerfull.