This is interesting stuff, I have been using flash for most of my macro work, I have exchanged some post with James previously. I have tried the gun on manual at 1/8 power and provided the background is close and the day is bright the results are good. The background and light are however not always obliging so I have reverted to the following scenario:
I have been using the high speed sync setting on the flash gun and setting the gun to -1/3 down to more recently -1 stop. My logic has been that if it is a particularly bright day and the shutter speed is faster than 1/250 sec then the flash produces the goods and if the shutter speed is slower, well the flash still delivers.
Jon