Accessories like that are meant to be used when you are bouncing the flash off of walls and the ceiling. So if you did try to manually figure out the proper flash exposure settings, it would be a big head ache. But luckily, many flashes have auto settings...many of them are TTL (through the lens), which usually gives pretty good results.
So I'd suggest that you keep your flash in auto (TTL) mode when shooting indoors with your Fong Dong. You can use FEC to adjust the flash exposure up or down from the auto value.
Personally, I like to have the camera in Manual mode when shooting with flash. With the flash in auto (and provided that the flash is dedicated to the camera), the flash will set it's output for the aperture and ISO that you have set. That leaves you with the shutter speed, which you can use to adjust the amount of ambient exposure that you want.