Calculating exposure with flash

Can someone explain if and how the power settings are used on the back of an alienbees flash to calculate the exposure you need.

On my canon flash I would typically meter the background in camera and then meter the subject and set the flash a accordingly to adjust. The flash however has settings that are in increments of stops not a fraction of power.

I may just be going about this completely wrong as well. This is my first monolight and I'm still trying to figure it out.

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