Welcome JKT! For the sake of mention, if you go with Manual mode and ambient light is a factor in your shot, ISO values aren't "arbitrary": they influence how bright/dark the ambient is just as much as shutter or aperture settings. I'm not sure about the 430II, but I believe some of the newer Canon flashes interpret their color temperature and report that to the camera, so if you have the camera on auto WB it'll be "calibrated" to the flash and unaware of the ambient light.