[jfrancho]

The meter is useful to determine ambient, or background exposure, which will be controlled by ISO/Av/Tv settings. The fact that P mode sets a standard exposure every time is why you want to use M. You want to control the ambient or background, not the camera. The TTL flash exposure (depends on brand) will determine the flash output, and can be adjusted using flash exposure compensation, FEC. When you bounce the flash, the light is scattered and diffused, and less intense. Some flashes will auto compensate for this, some won't, and I find that +2/3 FEC is what it takes to overcome it. What you have to realize is that shutter and aperture don't contribute as much to the subject exposure as it would be without a flash. Seperate the two functions in your head, and you'll be able to control your flash photography.