20D + 550ex flash settings

[Liverpool Steve]

Can anyone explain how the following settings for a 20d+550ex actually works. I've tested numerous settings and the results are OK. The reason why I ask is because if I take a meter reading indoors and say it gives me 1/15 @ 5.6, flash on auto, the results are sometimes poor. The settings below seem to work 99% of the time.

Camera in M mode. Then I will choose my f stop.. say f-8 then choose my time exposure based on my focal length, so 100 mm would be 1/100 or a little more. 550 ex I don't touch the FEC

I'm asking the question because I shoot weddings and will test this method further and if it proves to be reliable, then stay with it.
Weddings don't always allow you time to sit and think about what if?

Thanks
Steve[/u]