internal & external guide number

I have just purchased an Olympus 8080 digital camera. I do alot of picture taking at my church and school, in large auditoriums and meeting rooms, and I am tired of getting dark backgrounds when just using the built-in flash. I therefore am going to be using both the internal flash along with the Olympus FL-20 external flash (FL twenty)

If operating in manual mode, how does one figure the flash guide number when using both built-in and external flash?

I think maybe I answered this on another forum here?

Anyway, the camera's built-in flah shouldn't affect the output of the FL-20 at all. Unless there's something magical about the built-in flash, it doesn't have enough power to reach as far as the FL-20, so it won't have any affect on your exposure. Once you put the FL-20 on the hot shoe, just ignore the built-in flash.