If you are using TTL, the camera/flash themself will determine that for themself. Most TTL system will have the flash firing a pre-flash. The camera will read pre-flash through-the-lens (TTL) and tells the flash how much power to use.
If you are trying to do it manually, use a light meter.
From your thread title, you can do:
1) Shorten your distance from camera to subject.
2) Choose not to high ceiling (if you bounce to ceiling).
3) Increase your camera's ASA/ISO.
4) Use big aperture on your lens.
5) Use multiple flashes.
The basic you need to know: Use manual flash mode to calculate lens' aperture from GN number of the flash. Aperture = flash GN (ASA/ISO m/ft)/distance (in m/ft). The distance from camera/flash is calculate from camera/flash bounced from ceiling (if ceiling is your bouncer), and open aperture 1 or 2 stops more depend on ceiling height and color. And, finally trial shoot and look the result on preview lcd. Good if you know how to analyze the histogram.
Hmmmm...., use TTL mode flash will calculate the flash power for you in easy too :think: .