Okay, I'll have a go at this in technical terms. The main assumption is that you are planning to use this on your F80/N80 body (as indicated on your profile).
First to get the correct aperture without the diffuser.
The F80's built in flash has a Guide Number (GN) of 12 metres or approximately 40 feet at ISO 100. The basic equation to work out the correct aperture to get the correct exposure for a given distance in feet is:
Aperture =(GN x Film Speed Multipler)/Distance
eg. for a ISO 200 film (FSM = 1.4) for a subject at 10 feet (approx. 3 metres).
(40 x 1.4)/10 = f/5.6
This now determines the aperture to set on your lens in manual or aperture-priority mode. However, if you are using the program mode, the camera CPU should do this for you.
By adding on the diffuser, you are effectively enlarging the light source and spreading the light to cover a larger area. Unfortunately, this light spreading exercise will give you a minimum of a 2-stop loss (possibly upto a 3-stop loss). So that means that you will need to stop down to a least f/2.8 to compensate for a 2-stop loss (or f/1.4 for a 3-stop loss) or reduce your shutter speed by the required stops for the above mentioned calculation example.
So the only way out of this if you do not have fast lenses or cannot reduce your shutter speed anymore (without using a tripod) is to either get closer to your subject (reduce distance) or use faster film (increase FSM).
Having said all that, the main usage of the SG-2 diffuser is to reduce the effect of the built-in flash on the F80 (or F65 which has the same GN) to a minimum (as shown in the above example) and use an off-camera flash, say a SB-28 (GN 50m or 164ft max. at 85mm and 1/250th) or a SB-25 (GN 24m or 77ft max. at 85mm and 1/250th), attached to the SU-4 as the main flash. See attachment for example.
So... in short, yes you should be able to use the diffuser but you'll need to watch out for the items above our you will end up to under-exposed shots.
Just for you reference, here are some FSM for the more commonly use film speeds.
ISO 50 - 0.7 ISO 100 - 1.0 ISO 125 - 1.12 ISO 200 - 1.4 ISO 400 - 2.0 ISO 800 - 2.8 ISO 1600 - 3.95