The best thing is for indoor flash is to get a light meter, there are a few that are very good and not to expensive now.
For flash is just about a must. Some will read the flash, with the meter near your subject, and some will fire the flash and read your subject. As for outdoor, use the meter on your camera, your meter will work in the manual mode, if you don't know how, or have the manual, try downloading it from the Nikon site. Hope this helps.